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[ Option D ]

In C programming, keywords are special words that have a fixed and predefined meaning, decided by the language itself. We cannot change their meaning, nor can we use them as identifiers.

• The keywords are also known as reserve words.
• The meaning of each keyword is fixed.
• In ‘C’ language there are 32 keywords.
• We cannot use keywords as an identifier.
• All keywords are written in small letters.
   

[ Option B ]

A variable is a named location in memory that is used to hold or store the value. The value of variable can be modified in the program by the instruction. The variable names must follow the rules of identifiers.
RULES FOR WRITING IDENTIFIERS:

• First letter must be an alphabet or underscore.
• From second character onwards, any combination of digit, alphabet or underscore is allowed.
• Other than underscore no any special symbol is allowed.
• Keywords cannot be used as identifiers.

[ Option B ]

In programming like C, we store information using variables or other fixed items. Some values need to stay the same forever during the whole program run, like math number PI, number of days in a week, or maximum speed limit etc.
A constant is a value that is fixed and cannot be changed during the execution of a program. Constants are used when we want to make sure a value remains the same throughout the program.

#include <stdio.h>
int main() 
{
    int age = 29;// Variable.
    printf("Age : %d ", age);
    age = 34; // Changing value of variable.
    printf("Updated Age : %d ", age);
    const float PI = 3.14;// Constant.
    printf("Value of PI: %.2f ", PI);
    PI = 3.1415; // Error. Cannot change a constant.
    return 0;
}

[ Option D ]

A comment is used to add notes or explanations within the source code to make it easier for humans to understand. Comments are ignored by the compiler, so they do not affect the execution of the program.
In C, a multi-line comment starts with /* and ends with */. Single-line comments can also be written using //.

[ Option B ]

In C language the if is keywords. A keyword is a reserved word that has a predefined meaning in the language and cannot be used as an identifier. The if is used for decision-making.

[ Option C ]

The backslash constant is called the carriage return. It moves the cursor back to the beginning of the current line, without moving to a new line or erasing any existing content. This allows the next characters to overwrite the existing characters on the same line.

[ Option D ]

Must Know:

• ANSI C (C89/C90): The original standardized C (1989/1990) has 32 keywords.
• C99: A standard introduced in 1999 that added new features like variable-length arrays, inline functions, new data types, and new keywords.
• C11: A standard introduced in 2011 with improvements for concurrency, memory alignment, and multithreading, also adding new keywords.
• C99 adds 5 new keywords namely, _Bool, _Complex, _Imaginary, inline, restrict.
• C11 adds 7 new keywords namely, _Alignas, _Alignof, _Atomic, _Generic, _Noreturn, _Static_assert, _Thread_local
• So, C89/C90: 32, C99: 32 + 5 = 37 and C11: 37 + 7 = 44 keywords.

[ Option C ]

The keyword const is used when we want to make sure that the value of a variable cannot be changed after it is assigned. When a variable is declared with const, it becomes a constant, meaning its value remains the same throughout the program.

[ Option D ]

In C language, the header file math.h contains the prototypes of mathematical functions such as sqrt(), pow(), sin(), cos(), and log(). By including this header file at the beginning of a program using #include<math.h>, we can use these predefined functions to perform various mathematical calculations.

[ Option C ]

Combination of letters becomes an identifier. Identifiers are used for give the names of various program elements like variables, array, functions, structure etc.

RULES FOR WRITING IDENTIFIERS:

• First letter must be an alphabet (A to Z or a to z) or underscore (_).
• From second character onwards, any combination of digit, alphabet or underscore is allowed.
• Other than underscore no any special symbol is allowed.
• Keywords cannot be used as identifiers.

[ Option A ]

An identifier is the name we give to various program elements like variables, constants, functions, and other user-defined items. However, keywords cannot be used as identifiers because of meaning of each keyword is fixed or already known by the compiler.

[ Option B ]

The keywords are special words that have a fixed and predefined meaning, decided by the language itself.

• The keywords are also known as reserve words.
• The meaning of each keyword is fixed.
• In ‘C’ language there are 32 keywords.
• We cannot use keywords as an identifier.
• All keywords are written in small letters.

The main is the name of the function where program execution begins, but it is not reserved by the C compiler as a keyword.

[ Option A ]

The backslash constant  (backspace) moves the cursor one position back in the current line. It does not erase or delete the character. It only shifts the cursor position so that the next character printed will overwrite the previous one.
The C language defines several backslash character constants which are used for special purpose. They are called so because each backslash constant starts with backslash (). These are also called white spaces characters or escape sequence characters.