What single entry can be made after realizing that a payment was incorrectly classified as Notes Payable instead of Accounts Payable?

The correct entry to make after realizing that a payment was incorrectly classified as Notes Payable instead of Accounts Payable is to debit Notes Payable and credit Accounts Payable for the same amount, in this case, $2,000.

This entry corrects the classification error by decreasing the Notes Payable account, effectively removing the incorrect liability, and increasing the Accounts Payable account, reflecting the correct liability. Since both accounts represent liabilities, the debiting of Notes Payable reduces the company's obligations while the crediting of Accounts Payable increases its liabilities, accurately recording the current financial situation.

The rationale for the other options is that they do not correctly address the need to reclassify the payment from one liability to another. Specifically, options that involve Cash would imply an actual cash transaction, which is not taking place in this correction; this is purely a reclassification between liability accounts. Therefore, to properly adjust the accounts without affecting cash or introducing any unwarranted changes, the entry to correctly reflect the payment as an Accounts Payable is the most suitable action.