What should come in place of the question mark (?) in the given series?
21, 57, 87, 111, 129, ?

Given: 21, 57, 87, 111, 129, ?

Logic: Differences decrease by 6 each time.

57 - 21 = 36

87 - 57 = 30

111 - 87 = 24

129 - 111 = 18

Differences:

36, 30, 24, 18, 12 → decreasing by 6

Next term:

129 + 12 = 141

So, the missing term is 141.

Thus, correct option is (d).