Classification (letter pairs – fixed forward jump): In three pairs, the second letter is 3 positions ahead of the first in the alphabet; one pair violates this +3 rule. Identify the odd pair.

Introduction / Context:
Alphabet-pattern classification often uses fixed positional jumps. Here, test whether the right letter is exactly three steps ahead of the left one (A=1, B=2, …, Z=26). Three pairs obey the +3 rule; one does not. The calculation is light and intended to reward accuracy over guesswork.

Given Data / Assumptions:

• R→U: 18→21 = +3.
• B→E: 2→5 = +3.
• F→I: 6→9 = +3.
• I→M: 9→13 = +4 (violation).

Concept / Approach:
Convert letters to positions and compute the delta. Select the pair that does not yield +3. This is a standard technique for letter-series items and is quick to execute once the mapping A=1…Z=26 is internalized.

Step-by-Step Solution:

Verification / Alternative check:
Rebuild by starting letter: add three letters forward; if the result mismatches the given right letter, that pair is the outlier.

Why Other Options Are Wrong:

Common Pitfalls:
Off-by-one counting (forgetting to exclude the starting letter when stepping). Always count moves, not letters listed.

Final Answer:
IM