What was the most significant effect of the depression on American farmers? lack of fertilizer increased immigration rising transportation costs falling prices

The correct answer is - falling prices.

The American farmers were heavily affected by the Great Depression. The effect was mostly in the dramatic drop of the prices of their products. This lead to very low, in some cases almost no profit, because the prices went so low that what the farmers were getting for their products was enough to cover up their expenses.

The reason why this happened was that the farmers produced much more than what was needed on the American market, or the amount they could have sold abroad. In order to solve the problem, the government suggested to the farmers to lower their production so that they do not produce more than what the market can accumulate.