A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
GHI,?, TUV, BCD

Let's examine the first letters of each 3-letter cluster to understand the sequence gap:
First letter of 1st cluster = G (7)

First letter of 3rd cluster = T (20)

First letter of 4th cluster = B (2, or 28 in cyclic sequence)

Let's check the intervals between positions:

G (7)
+6

​
M (13)
+7

​
T (20)
+8

​
B (28)

This confirms that the missing first letter must start at position 7+6=13, which corresponds to M.
Since the letters within each cluster are consecutive alphabets (G-H-I, T-U-V, B-C-D), the missing cluster starting with M must be MNO.