The answer is {1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15}.
(A ∪ B) means the union of sets A and B, which is the set of all elements that are in either set A or set B or both. Therefore, (A ∪ B) is {1, 3, 5, 7, 9, 12, 15}.
Then, we take the union of (A ∪ B) and set C, which is the set of all elements that are in either (A ∪ B) or set C or both. Therefore, the final answer is {1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15}. This is because all the elements in A, B, and C are present in the final set.