What indicates the buyer's agreement to put the property up as collateral for a loan?

The mortgage is the correct answer because it is a legal document that establishes the buyer's agreement to use the property as collateral for a loan. When a buyer takes out a mortgage, they grant the lender the right to take possession of the property if they fail to make the required loan payments. This agreement creates a security interest in the property, ensuring that the lender has a claim against the asset should the loan default occur.

In contrast, a lien may indicate a claim against the property for debt owed, but it does not specifically refer to the borrower's agreement to use the property as collateral. The bill of sale is used to transfer ownership of personal property rather than real estate. The note, or promissory note, represents the borrower's promise to repay the loan but does not establish collateral. Thus, the mortgage is the appropriate document that showcases the buyer's consent to the property being collateral for the loan.