Overdraft protection helps cover a purchase in your checking account when...

Overdraft protection is a service offered by banks that allows you to complete a transaction even if you don't have enough funds in your checking account to cover it. When a purchase exceeds your available balance, overdraft protection can cover that difference, thereby preventing a declined transaction. This feature essentially provides a line of credit that temporarily allows the account holder to go into a negative balance up to a certain limit, ensuring that essential purchases can still be made.

The other options do not accurately describe the function of overdraft protection. A scenario involving excess funds isn't relevant, as overdraft protection is specifically designed for situations where the available balance is insufficient. Forgetting a bank login doesn't relate to the available balance and affects account access but not transaction coverage. Lastly, having already closed the account means no transactions can occur, making it irrelevant to the purpose of overdraft protection.