Think of it like this:
you have 4 objects (the 4 answer options, a, b, c, and d)
You are going to chose a group of 5, and the order matters (your answer to question #1 is independent of your choice on #3)
Repetition is allowed (you can answer "c" on all of the questions if you want to)
This is a simple permutation. Use the formula:

where n is the number of objects (the number of answer choices)
and r is the number of objects you will choose (5, one answer for each question)
To solve:

your answer!
Hope this helps! Feel free to ask any follow up questions, and please "vote up" :)