Explanation:

- According to the Indian Partnership Act, an agreement that restrains a continuing partner from carrying on any business, other than the business of the firm, while he remains a partner is considered valid.

- This restriction helps protect the interests of the firm and prevents conflicts of interest.

- Such restrictions are not against public policy, as they are connected to the business relationship in the partnership.

- Options explained:

- Option 1: Void — The agreement is not void, as it supports firm interests.

- Option 2: Voidable — It is not up to a party to void it; it is enforceable by law.

- Option 3: Valid — This is correct.

- Option 4: Unlawful — The law allows such agreements in partnerships.