To be a triangle, the two smallest sides added together have to be greater than the largest side.
A wouldn’t work, because 11 + 21 = 32, which is smaller than 33.
B wouldn’t work either, because 14 + 27 = 41, which is smaller than 43
C would work, because 11 + 13 = 24, which is larger than 23
D wouldn’t work, because 5 + 8 = 13, which is smaller than 15.
C is the only one that works, so the answer is C.