Which document serves as evidence of debt, indicating who owes what to whom?

The correct choice is the promissory note because it is the legal document that outlines the terms of a loan and serves as evidence of the debt. It specifies the borrower (the party who owes money), the lender (the party to whom the money is owed), the principal amount of the loan, the interest rate, the repayment schedule, and any other terms related to the obligation.

In contrast, a deed transfers ownership of property, but it does not itself indicate any debt or obligation. A mortgage is a security instrument that ties the promissory note to the property, giving the lender a claim against the property if the borrower defaults on the loan, but it does not serve as the primary evidence of the debt. Finally, a purchase agreement relates to the sale of the property itself and outlines the terms of the sale between buyer and seller, but it does not provide evidence of any debt associated with financing the purchase.