The effect of the Dust Bowl on the American farmer could BEST be described as...

D.They lost their jobs as businesses lost money and closed.

Explanation:

The Great Depression that started in 1929 affected most if not all Americans regardless of if they had invested in the stock market or not.

This was because banks lost millions when the stock market crashed because they had invested millions either directly or indirectly into stock.

Businesses therefore lost their deposited money in the banks and banks demanded that businesses owing should pay as they hoped to recoup some of their lost money.

This forced businesses to close and so people lost their jobs.