Explanation:

- Option 1: Implied or Express: A proposal can be communicated through words (express) or through actions and circumstances (implied).

- Option 2: Specific: Proposals can be tailored for a particular individual or scenario, detailing unique terms and conditions.

- Option 3: Generic: Sometimes, proposals aren't for a specific individual or group but are open to anyone who wishes to accept them.

- Option 4: Any of the above: ?? This is correct. Proposals can indeed be any of the above - implied, express, specific, or generic.