Explanation

Let the original side be x cm. The enlarged square has side (x + 8) cm, so (x + 8)² = x² + 120. Expanding: x² + 16x + 64 = x² + 120, which simplifies to 16x = 56 and x = 3.5 cm. A quick check: (3.5 + 8)² = 132.25 = 12.25 + 120, consistent. The answer is deliberately small — the 120 cm² increase comes mostly from the 8 cm border, not from x itself. Hence option (B) 3.5 cm.