It is true that any interpretation that can be defended with evidence from the text is a valid topic for a critical essay.
I think that C, that is, "they found the charge and trouble very great, and they had little or no crop it is not doubted", is your answer.
Understatement represents something as smaller or less intense than it reallly is, it presents it as less important. In sentence C, the speaker refers to a problem as a minor inconvinience "(...)trouble very great". Generarlly, we all know, that troubles are far from great. "They had little or no crop it is not doubted", you could change the focus and say that you have "some crop" instead of referring to the crop as being little.
