Select the option that is related to the fifth letter-cluster in the same way as the second letter-cluster is related to the first letter-cluster and the fourth letter-cluster is related to the third letter-cluster.
ISLAND : ANDISL :: EITHER : HEREIT :: IMPACT : ?

The correct option is ACTIMP.

To determine the missing letter-cluster, let us analyze the logical pattern behind the transitions in the given pairs:

1. First Pair: ISLAND : ANDISL
The word ISLAND consists of 6 letters. Let us divide it into two equal halves of 3 letters each:
- First half: ISL
- Second half: AND
By swapping the positions of the two halves (Second half followed by First half), we obtain the second cluster: ANDISL.

2. Second Pair: EITHER : HEREIT
Following the same logic for the 6-letter word EITHER, we divide it into two equal halves:
- First half: EIT
- Second half: HER
Swapping the positions of these halves (Second half followed by First half) gives the fourth cluster: HEREIT.

3. Third Pair: IMPACT : ?
Applying the exact same rule to the word IMPACT:
- First half: IMP
- Second half: ACT
Swapping the two halves (Second half followed by First half) yields the final cluster: ACTIMP.

Therefore, the letter-cluster related to IMPACT is ACTIMP.