Explanation:

- Option 1: A contract

- For a contract to be valid, acceptance must typically follow the mode prescribed by the offeror.

- If the offeror decides to keep quiet despite the incorrect acceptance, it implies implicit acceptance.

- A mutual agreement exists, forming a valid contract.

- Correct Answer: A contract.

- Option 2: No contract

- If acceptance isn't as prescribed, normally, no contract is formed.

- The contract depends on the offeror's response or objection.

- Option 3: A voidable contract

- A contract is voidable in instances of duress or misrepresentation, not in acceptance method issues.

- Option 4: An unenforceable contract

- An unenforceable contract is valid but cannot be enforced due to some legal technicality, like lack of formality.