Explanation
Correct answer: B.
Choice B is the best answer. The text tells a story of a first impression that turned out to be wrong: a serious American savant was dismissive of a goofy-looking, wisecracking guest at a British party, and then was shocked to learn that the guest was actually a prominent British chemist and inventor.
Why the other choices are wrong
Choice A
Choice A is incorrect. This is too strong and too narrow. Only at the very end is the savant "thrown aback" by the fact that the man was Sir Humphry Davy—he’s not "embarrassed about his own behavior."
Choice C
Choice C is incorrect. This isn’t the main purpose. The text never provides the history of how Sir Humphry Davy came to be famous. Nor does it provide any history for the American savant.
Choice D
Choice D is incorrect. This is too narrow. It doesn’t include the second half of the text, where the savant gets into a conversation with the man and then finds out that the man is Sir Humphry Davy.