Which of the following numbers will replace the question mark (?) in the given series?
18, 71, 21, 76, 24, 81, 27, 86, 30, ?

Given: 18, 71, 21, 76, 24, 81, 27, 86, 30, ?

Odd positions:  increasing by 3 each time.

Even positions: increasing by 5 each time.

Odd positions (starting with 18):
18 + 3 = 21
21 + 3 = 24
24 + 3 = 27
27 + 3 = 30

Even positions (starting with 71):
71 + 5 = 76
76 + 5 = 81
81 + 5 = 86
86 + 5 = 91

So, the missing number is 91.

Thus, correct option is (c).