The area of an equilateral triangle with side s is A = (√3/4)s². Substituting s = 20 cm: A = (√3/4)(20)² = (√3/4)(400) = 100√3 square cm. Option A (200√3) would be for a triangle with side 28.28 cm, Option C (50√3) would be for side 14.14 cm, and Option D (400) would be for an equilateral triangle with approximately side 30.3 cm or could be the result of forgetting the √3/4 factor.