Which of the following words is closest in meaning to “Collateral”?

Collateral means security pledged for a loan.

• Correct answer - security for a loan: it has the same meaning as Collateral.
• key: a device that opens a lock - wrong, it does not mean the same as Collateral.
• proposal: a suggestion put forward - wrong, it does not mean the same as Collateral.
• secret agreement: a private pact - wrong, it does not mean the same as Collateral.