Explanation:

In programming, a constant is a value that cannot be altered or modified during the execution of a program. When an argument from the parameter list of a function is defined as constant, it means that the value of that argument cannot be changed within the function.

Reason:

The reason behind this is that when a constant argument is passed to a function, it is essentially being passed by value. This means that a copy of the value is being made and passed to the function, rather than a reference to the original value. As a result, any modifications made to the argument within the function will only affect the copy and not the original value.

Consequence:

As a consequence, if an attempt is made to modify a constant argument inside the function, it will result in a compilation error. The compiler will detect the attempt to modify a constant value and raise an error, indicating that the modification is not allowed.

Benefits:

Defining an argument as constant provides several benefits. It helps ensure the integrity of the original value by preventing unintentional modifications. It also allows the programmer to easily identify which arguments are not intended to be modified within the function, improving code clarity and readability.

Example:

Let's consider a simple example to understand this concept better:

```
void modifyValue(const int x) {
x = 5; // This will result in a compilation error
}

int main() {
int num = 10;
modifyValue(num);
return 0;
}
```

In this example, the function `modifyValue` takes an argument `x` which is defined as a constant. When we try to modify the value of `x` inside the function, a compilation error will occur. The error message will indicate that the modification of a constant value is not allowed.

Therefore, option B is the correct answer: It cannot be modified inside the function.