Answer: A) complement
Step-by-step explanation:
Let's say we had the universal set of all natural numbers {1,2,3,4,...}
Then let's further say that a subset of this is the set of positive even numbers {2,4,6,8,...}
The complement of the second set would be the set of positive odd numbers {1,3,5,7,...} which is the stuff not found in the second set mentioned, but it's found in the universal set. This is one example of the complement of a set.