Choose the odd number: Among 13, 17, 19, and 27, three are prime numbers (having exactly two positive divisors), while one is composite. Identify the composite number that breaks the pattern.

Introduction / Context:
Prime-versus-composite identification is a common classification theme in verbal–reasoning number sets. Here, three entries are primes; exactly one is composite, and that is the odd one out.

Given Data / Assumptions:

• Candidates: 13, 17, 19, 27.
• Prime means exactly two positive divisors (1 and itself).
• Composite means more than two positive divisors.

Concept / Approach:
Check simple divisibility. All even numbers greater than 2 are composite; numbers ending with 5 (and greater than 5) are composite; digit-sum multiples of 3 indicate divisibility by 3. Use these filters, then confirm with small-prime checks when needed.

Step-by-Step Solution:

Verification / Alternative check:
Try direct factorization where obvious: 27 factors as 3 * 3 * 3. No such factorization exists for 13, 17, or 19 using small primes, confirming their primality.

Why Other Options Are Wrong:
13, 17, and 19 each have only two divisors and are therefore prime; they match the intended pattern.

Common Pitfalls:
Assuming “odd = prime.” Many odd numbers (like 27) are composite; use divisibility tests.

Final Answer:
27 is composite and thus the odd number.