Explanation:

- In this case, A finds B's mobile phone and returns it. After that, B makes a promise to give A Rs. 500.

- Option 1: Agreement – An agreement is every promise or set of promises forming a consideration for each other. In this situation, there is a promise but not a mutual consideration.

- Option 2: Contract – A contract needs offer, acceptance, mutual consideration, and intention to create legal relations. Here, A has already performed the act before B’s promise. This is a past consideration, and unless there was a prior request, it isn’t a contract as per the Indian Contract Act, Section 25. This is NOT a contract.

- Option 3: Acceptance – Acceptance is when an offer is accepted, forming an agreement. Here, there is no prior offer from B.

- Option 4: Offer – An offer is when someone proposes to do something to another party. Here, B is making a promise after the act is done. There was no initial offer.

The correct answer is Option 1: Agreement.

- The promise is an agreement (social/moral promise), but not enforceable as a contract.