Hello there! The answer is A.
Well, right off the bat we can eliminate B since 17, 19, 21 are not consecutive (meaning they don't come right after each other, there are the numbers 16 and 18 in between them). Next, let's add up the values of A, C and D to see which equals 54.
A. 17 + 18 + 19 = 54.
C. 18 + 19 + 20 = 57
D. 16 + 17 + 18 = 51
This means that A is your correct answer, I hope this helps & have a great day!