Question

during the 1920s, people would buy stock on margin, which meant that they
bought it on credit.
paid cash for it.
paid in installments.
bought it on speculation.

Explanation:

Buying stock on margin in the 1920s involved borrowing funds from a broker to purchase stocks, which is a form of buying on credit. Paying cash is the opposite, installments refer to periodic payments for a purchase, and speculation is the motive of betting on price changes, not the definition of buying on margin.

Answer:

bought it on credit.