This question asks you about given information from the text. So, the right answer has to be directly supported by the text.
@ Emerson and Fuller is introduced as “other transcendentalists” whose philosophical writings would have been more in common with King’s idea (15-23)
Let's get into answer choices.
A) It wouldn’t be right to say their idea was “less typical” of transcendentalism. In fact, Emerson and Fuller represents more general ideas of transcendentalism, because the author said "in many other transcendentalist writings, including works by Emerson and Fuller, King would have found ideas more nearly akin to his own" (20-23)
B) The opposite is true. (26-29)
C) 26-29 says that all transcendentalists advocates for individual reform.
D) How popular it was was not the issue.
E) This is it. Considering there is big chunk of Thoreau's idea not in accordance with King's while Kings' idea in agreement with Transcendentalists (11-23), this is essentially what the author is saying. Correct