OK, first things first:
Line 9:
To declare an array, you need a type, name and size. In this case you have the type (question), but no name.
Furthermore, you shouldn't store the data in a 3D array of the question struct, since the question struct already stores the question and answers.
Line 2:
Except it doesn't , it only stores two strings - one for the question, and one for an answer, instead of one for the question and four for the answers (assuming there is always four possible answers to each question).
So back to line 9, just use a 1D array with 50 elements, each element on its own storing the 5 strings.
Line 19:
http://www.cplusplus.com/reference/string/getline/
getline takes two inputs, an istream and a string, the istream object being fin, your ifstream object. Also, you need to read 5 times, once for the question, and four times for the answer.
Line 15:
You're looping while fin is not at the end of the file, but within this loop you're reading SIZE times (presumably 50, right?) which would in itself read the entire file within one while loop, so it's redundent. It would be better to put the loop around the getline, and check instead to see if fin is good (while(fin.good){...}).
Line 24:
read_questions(...) returns void, not int, so you don't return a value, you just call return to exit the function. This is also another reason why your while loop is redundent - after reading SIZE times, the function returns, and never loops through the while again.
If I'm unclear about any of this, please tell me and I'll try to be more clear.