After the stock market crash,

A. banks had plenty of cash to repay depositors.

B. banks loaned more money to Americans.

C. Americans calmly invested more in the stock market.

D. Americans panicked and rushed to banks for their money.

D is the correct answer. When the stock market crashed, Americans were worried that they would lose money, and rushed to the banks to get their money while they could. The banks could not pay all of their depositors at once, and subsequently failed.