Which of the following is necessary for enforcing a mortgage?

To enforce a mortgage, it is essential to have a document that establishes a lien on the property. A mortgage is essentially a legal agreement that creates a lien—a claim against the property—allowing the lender to take possession if the borrower defaults on their loan. This lien is recorded in public records, which provides public notice of the lender's interest in the property.

Having a lien in place gives the mortgage holder the legal right to foreclose on the property if the borrower fails to meet the repayment terms. This is why the document establishing the lien is necessary for the enforcement of the mortgage. Without this document, there would be no legal basis for the lender to enforce their rights over the property in question.

The other choices, such as a registered deed restriction, a bona fide purchaser agreement, or a notarized lease contract, do not directly establish the necessary legal framework to enforce a mortgage. While they may play roles in real estate transactions or property management, they are not fundamental to the enforcement of a mortgage.