Find the missing number: 1,6,15, ? 45, 66, 91

A 28

B 27

C 26

D 25

Correct Answer: Option A

Pattern in the series is, +5, +9, .., .., +21, +25

So 4 is getting increased at every term addition.

Missing term in the series will be 15+(9+4)

= 15+13 = 28