Please don't take this down, I genuinely do not understand it!

Let X and Y be the following sets:

X = {5,10,15}
Y = {3,6,9,12,15}

Which of the following is the set X∪Y ?
CHOOSE ONE ANSWER
A. {3,6,9,12,15}
B. {5,10,15}
C. {3,5,6,9,10,12,15}
D. {}