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