The correct answer is A) investors purchased the stocks with little cash down.
Stock bought on margin was considered a risky investment because investors purchased the stocks with little cash down.
If investors buy stock with little cash, it is considered a risky investment in that if the price of the stock drops, it has to repay the loan. Of course, the investor is thinking that if the investment is high-risk, the return is going to be more attractive. But the risk part comes when this is not the case and the investor does not have enough money to pay the debt.