Final answer:
A bank run occurs when depositors rush to the bank to withdraw their money due to rumors of a bank's financial instability.
Step-by-step explanation:
A bank run occurs when there are rumors (possibly true, possibly false) that a bank is at financial risk of having negative net worth. As a result, depositors rush to the bank to withdraw their money and put it someplace safer. Even false rumors, if they cause a bank run, can force a healthy bank to lose its deposits and be forced to close. Deposit insurance guarantees bank depositors that, even if the bank has negative net worth, their deposits will be protected.
Therefore, the correct answer is a. A bank run.