Explanation:

- Option:1, is a contract

- A quasi-contract is not actually a contract.

- It is a legal substitute formed to impose equity between parties.

- Option:2, is an agreement

- It is not an agreement made between parties.

- Rather, it is imposed by the court to prevent unjust enrichment.

- Option:3, has only a legal obligation

- While it involves legal obligations, it’s not solely about them.

- It is intended to enforce an obligation to ensure fairness.

- Option:4, is none of these

- A quasi-contract is not naturally a contract, agreement, or solely legal obligation.

Option 4 is correct: it does not fit precisely as any of the other options.