A series is given with one term missing. Choose the correct alternative from the given ones that will complete the series.
ABC, PQR, DEF, STU, ?

The series consists of alternating groups of three consecutive alphabetical triplets:
1st triplet: ABC

3rd triplet: DEF (skipping PQR)

5th triplet: This should continue from DEF, which gives GHI.

Let's double-check the alternating set:

2nd triplet: PQR

4th triplet: STU

Thus, following the primary sequence (ABC→DEF→GHI), the missing term is GHI.