Hello!
The set X ∪ Y, can be found by combining the given sets of the sets into one set.
You are given set X = {5, 10, 15} and set Y = {3, 6, 9, 12, 15}.
∪ represents union, so you put these sets together.
Also, since the element 15 is in both sets, it should be in the X ∪ Y set, once.
Therefore the answer is A, X ∪ Y = {3, 5, 6, 9, 10, 12, 15}.