Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
XZYX, XYXX, XXWX, XWVX, ?

The correct option is XVUX.

To find the letter-cluster that replaces the question mark (?) in the given series, let us analyze the pattern in each letter position from left to right across the clusters:

Given series:
1. XZYX
2. XYXX
3. XXWX
4. XWVX
5. ?

Step 1: First letter position
In all the given clusters, the first letter is always X.
So, the first letter of the missing cluster is X.

Step 2: Second letter position
Let us observe the sequence of the second letters:
Z → Y → X → W
Here, each letter moves one position backward in alphabetical order:
Z (-1) → Y
Y (-1) → X
X (-1) → W
Following this pattern, the next letter is:
W (-1) → V.

Step 3: Third letter position
Let us observe the sequence of the third letters:
Y → X → W → V
Here, each letter also moves one position backward in alphabetical order:
Y (-1) → X
X (-1) → W
W (-1) → V
Following this pattern, the next letter is:
V (-1) → U.

Step 4: Fourth letter position
In all the given clusters, the fourth letter is always X.
So, the fourth letter of the missing cluster is X.

Combining all four positions together, we get the letter-cluster: XVUX.