Complete the decreasing-by-two letter series with missing letters: ZX_TR_NLJ_FD_. Choose the 4-letter string that correctly fills the blanks.

Introduction / Context:
This is a classic alphabet series problem featuring uniform backward steps. The letters descend by a fixed interval, and the task is to identify the exact missing letters that preserve the pattern from left to right.

Given Data / Assumptions:

• Visible skeleton: Z X _ T R _ N L J _ F D _
• We assume a consistent negative step size (backward movement) and standard alphabet order.

Concept / Approach:
Look for a constant difference between consecutive visible letters. From Z to X is −2 and from T to R is −2. If the rule is consistent, the entire chain should decrease by 2 each time. We then fill every blank using the same −2 step.

Step-by-Step Solution:
1) Start at Z (26). Apply −2 repeatedly: Z→X→V→T→R→P→N→L→J→H→F→D→B.2) Map to the skeleton: Z X V T R P N L J H F D B.3) Extract the missing letters in order of blanks: V, P, H, B.4) Concatenate: VPHB.

Verification / Alternative check:
Re-scan the finished sequence to ensure every adjacent pair is −2: Z→X→V→T→R→P→N→L→J→H→F→D→B. All intervals are correct, so the fill is validated.

Why Other Options Are Wrong:

• VRHB: The second letter R implies Z X V T R … which would skip P (violates uniform −2 at that location).
• UPJB: U is −1 from V (off by one), and P/J/B placements misalign the strict −2 rhythm.
• VPJB: J appears where H is required; again, interval mismatches occur.

Common Pitfalls:
Common errors include mixing a −1 or −3 step at one of the blanks or forgetting letters between large jumps (like R→N). Always chart the entire descent to keep the −2 consistency intact.

Final Answer:
VPHB