Final answer:
The correct function declaration for a function to get both the number of items and the cost per item from a user is 'void getData(int* count, float* cost)', which utilizes pointers to modify the values directly.
Step-by-step explanation:
A good function declaration to use if you need a function to get both the number of items and the cost per item from a user would be option B) void getData(int* count, float* cost). By using pointers as function parameters, we can directly modify the original variables that are passed to the function, effectively getting the user's input for both the number of items and the cost per item during the function's execution.
This approach is preferred because the typical use case for such a function is not just to perform operations within the function's scope but to modify values that need to be accessed outside of the function. In comparison, option A) would merely make a copy of the values, without a way to get the input back to the caller.
Option C) cannot retrieve both values since it can only return a single integer. Option D) is syntactically incorrect in C/C++ as functions cannot return multiple values without using a struct or pointers. Hence, the use of pointers in option B) is the correct choice to achieve the needed functionality.