Which letter cluster will replace the question mark ($?$) to complete the given series?
CHO, DIP, EJQ, ?

Let's analyze the positional shift for each letter of the cluster sequentially:
1. First Letter:
C $\stackrel{+1}{\to }$ D $\stackrel{+1}{\to }$ E $\stackrel{+1}{\to }$ F
2. Second Letter:
H $\stackrel{+1}{\to }$ I $\stackrel{+1}{\to }$ J $\stackrel{+1}{\to }$ K
3. Third Letter:
O $\stackrel{+1}{\to }$ P $\stackrel{+1}{\to }$ Q $\stackrel{+1}{\to }$ R

Combining all positions, the missing cluster is FKR.