Select the term that will come next in the following series.
A12B, D15E, G18H, J21K, ?

Given: A12B, D15E, G18H, J21K, ?

Logic: 1st letter + 3, 2nd number + 3 and 3rd letter + 3 place.

For, 1st letter

A + 3 = D, D + 3 = G, G + 3 = J, J + 3 = M

For, 2nd number

12 + 3 = 15, 15 + 3 = 18, 18 + 3 = 21, 21 + 3 = 24

For, 3rd letter

B + 3 = E, E + 3 = H, H  + 3 = K, K + 3 = N

So, the missing term is M24N.

Thus, correct option is (a).