Concept: A perfect square is the product of an integer multiplied by itself. Explanation: Check each option: 4 = 2×2, so it is a perfect square. 9 = 3×3, so it is a perfect square. 16 = 4×4, so it is a perfect square. 27 = 3×3×3, which is a cube, not a square. Thus, 27 is not a perfect square. Answer: Option D (27)