He relays most on Historical events
The answer is b, are.
The reason is because it is the only one that fits in the sentence that makes sense!
Thoreau uses the description of the movement of the trees and the serenity of the lake to support the idea that true reflection is achieved through solitude, ideally while in nature. Thoreau believes that a oneness with nature and with the self are essential for becoming a more enlightened person, and that solitude is key ingredient for this. Thoreau describes the serenity of the lake and its distance from the storm, much as he would describe the serenity of the mind when far from the hustle and bustle of society.
A would be correct. Think about it; plot contrivances are of the mistaken identity type, so they wouldn't help the mood. Historical accuracy wouldn't either, because its just who was where when. Anticlimax is basically a disappointing end to a story, and trust me, those definitely don't improve the mood. Appropriate comparison makes the most sense.
I hope this is right.