A

Explanation:

Contracts with an alien enemy (a person from a country at war with the contracting party’s country) are considered unlawful due to public policy, as they may undermine national interests during wartime. Such contracts are void and unenforceable. Option A (unlawful on the ground of public policy) is correct. Option B is incorrect, as the unlawfulness stems from public policy, not mere difficulty of performance. Option C (valid) and D (voidable) are incorrect, as these contracts are void, not valid or subject to cancellation. The answer “A” is as provided in the attachment.