A study of the causes of the American Revolution of 1776, the French Revolution of 1789, and the Russian Revolution of 1917 best supports the generalization that revolution is most likely to occur when

3 answers

The government doesn't treat its citizens right.
The causes are economic in all three, but famine played a large part in the latter two revolutions. In the Americas, one has to remember also a lot of people in America left Europe because they didn't like it there, ie, they were already fed up. Some states were actually populated with prisoners.

So your answer, I think is Ms Sue's, but I would add to hers "in tough economic times".
Agreed.