Explanation:

- Option 1: unlawful on the ground of public policy ??

Agreements with an alien enemy (person from a country currently at war with your own) are not allowed because it can harm the nation's welfare and security. Public policy forbids such dealings.

- Option 2: unlawful because it becomes difficult to be performed

The difficulty of performance is not the primary reason. It is banned mainly due to the public policy concern, not practical difficulty.

- Option 3: valid

These agreements are not valid. The law prohibits making agreements with alien enemies.

- Option 4: voidable

Voidable contracts are those which can be affirmed or rejected by one party, but contracts with an alien enemy are outright prohibited and void, not voidable.

Correct Answer: Option 1 is correct—agreements with an alien enemy are unlawful on the ground of public policy.