Fill in the blank to complete the given letter series.
\text{ABC, GHI, _____, STU}

Let's analyze the pattern of the first letters of each triplet in the series:
* $\text{A}\to \text{G}$: Position value changes from $1\to 7$ ($+6$)
* $\text{G}\to \text{?}$: Following the pattern $+6$, the next position should be $7+6=13$, which corresponds to the letter $\text{M}$.
* $\text{M}\to \text{S}$: Position value changes from $13\to 19$ ($+6$), which confirms the pattern.

Since the letters within each group are consecutive alphabetical sequences ($\text{A-B-C}$, $\text{G-H-I}$, $\text{S-T-U}$), the missing triplet starting with $\text{M}$ will be $\text{MNO}$.