Classification – Odd one out (four-letter series – monotonic order): Identify the string that is not strictly increasing alphabetically: PQXZ, BCQN, ABDF, MNPR.

Introduction / Context:
A common discriminator in letter quartets is whether the sequence of letters moves strictly forward in the alphabet. Three options list letters in left-to-right increasing order (though by unequal steps). One option breaks monotonicity by moving backward at the end.

Given Data / Assumptions:
Convert letters to positions A=1…Z=26 to inspect order:

• PQXZ → 16,17,24,26 (increasing).
• ABDF → 1,2,4,6 (increasing).
• MNPR → 13,14,16,18 (increasing).
• BCQN → 2,3,17,14 (increase, then decrease from 17 to 14).

Concept / Approach:
We only need strict monotonicity, not equal steps. Any backward move disqualifies the string. BCQN rises B→C→Q, then falls Q→N, hence it is the outlier.

Step-by-Step Solution:
1) Check each adjacent pair for forward motion.2) Identify the unique backward step: Q to N in BCQN.

Verification / Alternative check:
Even if PQXZ jumps by +7 and +2 at the end, it remains strictly increasing; thus it still conforms. Only BCQN violates monotonicity.

Why Other Options Are Wrong:
ABDF, MNPR, PQXZ each preserves A→Z order throughout.

Common Pitfalls:
Expecting equal step sizes. The test only demanded “increasing,” not “arithmetic progression.”

Final Answer:
BCQN