Answer:
The best answer would be "Each time he had pulled a twig he had communicated a slight agitation to the tree — an imperceptible agitation, so far as he was concerned, but an agitation sufficient to bring about the disaster".
Step-by-step explanation:
The second sentence is the best option because it sounds like a summary of the passage. Based on this passage, it is possible to state that it is a cold environment with no high winds, something that contributes to the fact that someone is under a tree with a bonfire. The person gets to pull out a twig from the tree, maybe to maintain the bonfire burning on. However, the person did not expect to make the tree agitate that much for dropping down all the snow accumulated in all boughs, which get to fall off one after the other, creating a huge snow landslide on the person and on the bonfire. Therefore, that movement put out the bonfire and replaced the flames with a lot of snow and ice, that is, an almost imperceptible movement becomes responsible for a disaster.