Classification – Odd one out (prime vs composite) Three of these odd numbers are prime. Identify the composite number and mark it as the odd one out.

Introduction / Context:
In sets of small odd numbers, prime recognition is straightforward. The presence of a clear composite among primes creates a simple odd-one-out decision.

Given Data / Assumptions:

• Options: 11, 13, 15, 17
• All are odd; primes are indivisible except by 1 and themselves.

Concept / Approach:
Check divisibility by 3 and 5 first. 15 has factors 3 and 5, hence composite. 11, 13, 17 are primes.

Step-by-Step Solution:
15 = 3 * 5 → composite.11 → prime.13 → prime.17 → prime.

Verification / Alternative check:
No other small prime divides 11, 13, 17. They remain prime under trial division up to their square roots.

Why Other Options Are Wrong:

• 11: Prime.
• 13: Prime.
• 17: Prime.
• None of these: One composite is present (15).

Common Pitfalls:
Assuming all odd numbers are prime. Many odd numbers, including 15, are composite.

Final Answer:
15