Classification – Alphanumeric codes: choose the odd one out by inspecting the numeric part. Three codes contain a composite even number (non-prime); one code contains the only prime number. Which code is different? Options: ZS12, PM4, RJ16, FD2.

Introduction / Context:
When multiple rules could exist among the letters, the number itself sometimes provides the cleanest separator. Here we test primality/compositeness of the numeric component to find the outlier.

Given Data / Assumptions:

• Codes: ZS12, PM4, RJ16, FD2.
• Numbers: 12, 4, 16, 2.
• Definition: a prime has exactly two positive divisors; composites have more than two.

Concept / Approach:
Classify each number as prime or composite and select the only prime as the odd one out.

Step-by-Step Solution:
12 = 2 * 2 * 3 → composite.4 = 2 * 2 → composite.16 = 2^4 → composite.2 → prime (the only even prime).

Verification / Alternative check:
Quick divisor check confirms only 2 has exactly two divisors (1 and 2).

Why Other Options Are Wrong:
ZS12, PM4, RJ16: Their numeric parts are composite, so they form the majority group.

Common Pitfalls:
Misclassifying 1 (neither prime nor composite) or overlooking that 2 is the only even prime.

Final Answer:
FD2