Explanation

The extended sine rule states that in any triangle, a/sin A = 2R, where R is the circum-radius. Here A = 30° and R = 10 cm, so BC = 2R sin A = 2 × 10 × sin 30°. Since sin 30° = 1/2, this gives BC = 20 × 1/2 = 10 cm. The result is exact — no approximation of π or radicals is involved. Hence option (B) 10 cm.