Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.
PQR, STU, VWX, ?

The correct option is YZA.

To find the missing letter-cluster that replaces the question mark (?), let us analyze the pattern followed by the letters in each position across the series.

Step 1: Analyze the first letter of each cluster

P → S → V → ?

Looking at their positions in the English alphabet:

P (16) + 3 = S (19)

S (19) + 3 = V (22)

V (22) + 3 = Y (25)

So, the first letter of the missing cluster is Y.

Step 2: Analyze the second letter of each cluster

Q → T → W → ?

Looking at their positions in the English alphabet:

Q (17) + 3 = T (20)

T (20) + 3 = W (23)

W (23) + 3 = Z (26)

So, the second letter of the missing cluster is Z.

Step 3: Analyze the third letter of each cluster

R → U → X → ?

Looking at their positions in the English alphabet:

R (18) + 3 = U (21)

U (21) + 3 = X (24)

X (24) + 3 = A (1, wrapping around the alphabet)

So, the third letter of the missing cluster is A.

Combining all the letters together, we get YZA.