The correct answer is D. Declaration.
A variable declaration statement defines a variable and its type. It does not indicate whether a variable is passed by value or by reference.
A variable passed by value is a copy of the original variable. Any changes made to the copy do not affect the original variable.
A variable passed by reference is a reference to the original variable. Any changes made to the copy also affect the original variable.
The following code shows how to declare a variable and pass it by value:
``` int x = 10;
void foo(int y) { y = 20; }
foo(x);
// x is still 10 ```
The following code shows how to declare a variable and pass it by reference:
``` int x = 10;
void foo(int &y) { y = 20; }
foo(x);
// x is now 20 ```
