Classification (numbers – perfect squares): Three numbers are perfect squares; one is not a perfect square. Identify the odd number.

Introduction / Context:
Number classification often tests recognition of perfect squares. Among the given numbers, three are perfect squares of integers; one is not. The task is to select the non-square number quickly and confidently.

Given Data / Assumptions:

• Known squares: 3^2=9, 4^2=16, 5^2=25.
• 18 is not an integer square (sqrt(18)≈4.2426...).

Concept / Approach:
Check each number against a memorized square table up to at least 20^2. Alternatively, factorize: a perfect square has even exponents in prime factorization.

Step-by-Step Solution:

Verification / Alternative check:
Take square roots: only 9,16,25 yield integers; 18 does not.

Why Other Options Are Wrong:

Common Pitfalls:
Misremembering squares around 20–30. Sticking to a simple checklist (9,16,25) prevents errors.

Final Answer:
18