The best answer here is A. If we refer to the text itself, we can see why. As the story progresses, the pigs start to think of themselves as superior to the other animals for one reason or another. In fact, they start to take on many of the characteristics of the humans they earlier touted as being evil and the reason for all their problems.
While the other animals might not have totally believed them, they went along with it because the society itself was equal. Each animal had its job to keep things running. However, because the pigs think themselves superior, they decide they should be the ones to rule and stop working. They also come up with a variety of rules that are supposed to exert their superiority over the other animals. However, at this point, there isn't much the other animals can do except grin and bear it and hope that it gets better.
The answer is, "however".
Force or oblige (someone) to do something.
The prevalent tone that this sentence conveys is sadness. The verb 'plodded' suggests exhaustion, 'dragging' implies hopelessness and 'empty night' could be a metaphor for their current perspective of their souls.
