The correct answer is A (customs union) because a common market represents the next level of economic integration, encompassing all features of a customs union while also allowing the free movement of factors of production among member countries.

• Option B (monetary union) is incorrect because a monetary union involves the adoption of a common currency, which is not a requirement for a common market.

• Option C (economic union) is incorrect because an economic union requires a higher degree of integration, including common economic institutions and coordinated policies, which go beyond the scope of a common market.