Select the letter-cluster from the given options that will replace the question mark (?) in the following series.
ABZ, BCY , CDX, DEW, ?

The correct option is EFV.

To find the missing letter-cluster in the given series ABZ, BCY, CDX, DEW, ?, let us analyze the pattern followed by each letter position across the clusters step-by-step.

1. Pattern for the first letter:
The first letters of the given clusters are:
A → B → C → D → ?
Each letter moves forward by 1 position in alphabetical order (+1).
$A+1=B$
$B+1=C$
$C+1=D$
Following this pattern, the first letter of the missing cluster will be:
$D+1=E$

2. Pattern for the second letter:
The second letters of the given clusters are:
B → C → D → E → ?
Each letter moves forward by 1 position in alphabetical order (+1).
$B+1=C$
$C+1=D$
$D+1=E$
Following this pattern, the second letter of the missing cluster will be:
$E+1=F$

3. Pattern for the third letter:
The third letters of the given clusters are:
Z → Y → X → W → ?
Each letter moves backward by 1 position in reverse alphabetical order (-1).
$Z-1=Y$
$Y-1=X$
$X-1=W$
Following this pattern, the third letter of the missing cluster will be:
$W-1=V$

Combining all three letters, we get EFV.