18 - 2n2 for n = 3 Need help

The permutations of a certain set are all the possible ways of arranging all the elements of the sets in all possible orders.

This is why option A is a permutation, you assign a position to each of the 5 runners, so you end up with the entire set of runners, ordered in some way.

Option B chooses only 2 students from a whole school (which we may assume counts more than 2 students..)

Option C chooses 4 books out of 30

Option D chooses 5 CDs out of 100