My calculator program keeps crashing at the end

[GCDEF]

If you're looking for opinions, I'd suggest a separate function that gets the numbers, and passing those numbers to the add, multiply functions. Any time you're doing the same thing repeatedly, you should be using a function that you call.

Why do you have calls like
else
multiplyA();
in all operation functions?

You know the operation type in whateva. Just call the appropriate function after enterB.