Which of the following letter-number clusters will replace the question mark (?) in the given series to make it logically complete?
FJN 45, DHL 40, BFJ 35, ZDH 30, XBF 25, ?

The correct option is VZD 20.

To find the missing letter-number cluster in the series, let us break down the pattern into two components: the letter cluster and the numerical value.

Step 1: Pattern in the numerical values
Looking at the numbers in the given series:
45, 40, 35, 30, 25, ?
Each consecutive number decreases by 5.
25 - 5 = 20
So, the number in the missing cluster is 20.

Step 2: Pattern in the letter clusters
Let us analyze the positional values of each letter in the English alphabet (where A = 1, B = 2, ..., Z = 26):
1st cluster: FJN → F (6), J (10), N (14)
2nd cluster: DHL → D (4), H (8), L (12)
3rd cluster: BFJ → B (2), F (6), J (10)
4th cluster: ZDH → Z (26), D (4), H (8)
5th cluster: XBF → X (24), B (2), F (6)

Now, let us examine the movement for each position across the clusters:
• First letter: F (6) → D (4) → B (2) → Z (26) → X (24) → ?
Each first letter moves backward by 2 steps (-2 in alphabetical order).
24 - 2 = 22, which corresponds to the letter V.

• Second letter: J (10) → H (8) → F (6) → D (4) → B (2) → ?
Each second letter moves backward by 2 steps (-2 in alphabetical order).
2 - 2 = 0 (or 26 in cyclic order), which corresponds to the letter Z.

• Third letter: N (14) → L (12) → J (10) → H (8) → F (6) → ?
Each third letter moves backward by 2 steps (-2 in alphabetical order).
6 - 2 = 4, which corresponds to the letter D.

Combining the letter cluster and the numerical value gives VZD 20.