The correct answer is B) insufficient rainfall.
The ecological factor that made agriculture difficult in the Great Plains in the 1930s was insufficient rainfall.
Insufficient rainfall in the Great Plaints caused havoc in the region in the time of the Dust Balls. This was a period of dust storms that had terrible effects on the fields and agriculture. Severe dry impeded good crops and the lack of water affected the farmers in the plains during the 1930s. It also affected animals in the farms. The dust storm killed livestock. That was the reason why many families decided to move to the West, looking for better living conditions.