The most accurate reason from the options provided is: Americans realized that business was not immune to failure and could collapse.
The excerpt speaks to a sense of stability and permanence associated with American business and prosperity, which was shattered by the stock market crash. The realization that even seemingly strong and dependable businesses could fail contributed significantly to the loss of confidence in the overall system. The crash revealed vulnerabilities in the economy and the fragility of the prosperity that many had taken for granted, leading to widespread disillusionment.