Final answer:
Both the functions 'trip(int *x)' and 'quad(int &x)' change the value of the variable passed to them, as they receive arguments by pointer and by reference, respectively.
Step-by-step explanation:
To determine which functions change the value of the argument passed to them, we need to examine the way each function is receiving its parameter. The function double(int x) takes an integer by value, which means it works on a copy of the original value and does not alter the actual variable that was passed in. In contrast, both trip(int *x) and quad(int &x) receive the variable by pointer and by reference, respectively. This implies that they directly work on the original value and have the ability to modify it.
trip operates with a pointer to an integer, thus changes made through this pointer will affect the original value. Similarly, quad functions with a reference to an integer, and modifications within the function will change the value of the original integer.
Therefore, the correct answer to which functions change the value of the variable passed to them is b) Trip and quad.