A function’s header includes the . . . . . . . . data type.

The correct answer is: A. As

A function’s header includes the return type. The return type is the data type of the value that the function returns. For example, the following function has a return type of int:

int add(int a, int b) {
return a + b;
}

The return type is specified after the function name and before the parentheses. It is important to specify the return type of a function, as this tells the compiler what type of value the function will return.

The other options are incorrect because they are not data types. As is a preposition, To is a preposition, From is a preposition, and By is a preposition.