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[ Option C ]

In programming, we often need to write the same logic again and again for different data types. For example, a function to add two integers and another function to add two floating-point numbers. Writing separate functions for each data type leads to code duplication.
To solve this problem, C++ provides the concept of generic programming using the template keyword. 
A template allows us to write one function or one class that can work with any data type. The actual data type is decided at compile time when the program is compiled.
Thus, the template keyword is mainly used for developing generic programming, which helps in:

• Writing type-independent code.
• Reducing repetition of code.
• Increasing reusability and maintainability.

#include<iostream>
using namespace std;
template<class T>
T add(T a,T b)
{
    return a+b;
}
int main()
{
    cout<<add(2,5)<<endl;
    cout<<add(2.5,6.3)<<endl;
    return 0;
}
OUTPUT
7
8.8

• In C++, creating reference is done using reference variable, not template.
• The template provides reusability, but the main purpose is generic programming.

[ Option D ]

The original function template is declared for one argument as:
template <class X>
void f(X a) {
    cout << "Inside ";
}
In main(), the code calls:
f(10);      // Calls the first template function.
f(10, 20);  // Needs a new template function for two arguments.
The second call f(10, 20) cannot use the first template because it only takes one argument. Therefore, we need to overload the template for two arguments of possibly different types as:
template <class X, class Y>
void f(X a, Y b) {
    cout << "Outside ";
}