The correct answer is B) bought stock on credit, thinking that prices would continue to rise.
What investors did that helped trigger the stock market crash in 1929 was "they bought stock on credit, thinking that prices would continue to rise."
And that did not happen.
The Great Depression started on October 29, 1929, after the crash of the US stock market. Thousands of people lost their jobs, many companies closed, and many banks went into bankruptcy. Those were terrible economic years in which people tried to survive the harsh economic conditions.