Which of the following letter-clusters will replace the question mark (?) in the given series?
ICE, LGJ, OKO, ?

To find the letter-cluster that will replace the question mark (?) in the given series, we need to analyze the pattern of each letter position in the clusters:

The given series is:
1st cluster: ICE
2nd cluster: LGJ
3rd cluster: OKO
4th cluster: ?

Let's look at the pattern for each position of the letters in the clusters:

1. First letter of each cluster:
From I to L: $\text{I}+3=\text{L}$ (I is the 9th letter, L is the 12th letter)
From L to O: $\text{L}+3=\text{O}$ (L is the 12th letter, O is the 15th letter)
Following this pattern, the next first letter should be:
From O to the next letter: $\text{O}+3=\text{R}$ (O is the 15th letter, R is the 18th letter)

2. Second letter of each cluster:
From C to G: $\text{C}+4=\text{G}$ (C is the 3rd letter, G is the 7th letter)
From G to K: $\text{G}+4=\text{K}$ (G is the 7th letter, K is the 11th letter)
Following this pattern, the next second letter should be:
From K to the next letter: $\text{K}+4=\text{O}$ (K is the 11th letter, O is the 15th letter)

3. Third letter of each cluster:
From E to J: $\text{E}+5=\text{J}$ (E is the 5th letter, J is the 10th letter)
From J to O: $\text{J}+5=\text{O}$ (J is the 10th letter, O is the 15th letter)
Following this pattern, the next third letter should be:
From O to the next letter: $\text{O}+5=\text{T}$ (O is the 15th letter, T is the 20th letter)

Combining the letters we found, the next cluster in the series is ROT.

Therefore, the correct answer is ROT.