A series is given with one them missing selected the correct value from the given ones that will complete the series.
A15N, I31V, Q21D, Y37L,?
The pattern is:

And the number between the alphabets is the sum of position of alphabets in the general alphabetic series.

A + N = 1 + 14 = 15
I + V = 9 + 22 = 31
Q + D = 17 + 4 = 21
Y + L = 25 + 12 = 37
G + T = 7 + 20 = 27
Hence, the answer is 'G27T'.
