Choice C is the best answer because it most logically completes the text’s discussion of the influences on Banisadr’s work. As used in this context, “unimportant” means trivial or lacking value. “It is by no means” establishes that the word that goes in the blank is contradicted by other information; the material that follows “indeed” later in that sentence provides the contradicting information—namely, that Banisadr himself cites Bosch as an inspiration. In other words, the sentence indicates that Bosch’s influence on Banisadr is significant, and thus recognizing that influence is by no means unimportant.
Choice A is incorrect because it wouldn’t make sense to say that recognizing Bosch’s influence on Banisadr isn’t “substantial,” or meaningful. The text states that Banisadr himself cites Bosch as an influence. Choice B is incorrect because it wouldn’t make sense to say that it isn’t “satisfying,” or pleasing, to recognize Bosch’s influence on Banisadr. The text states that Banisadr himself cites Bosch as an influence. Choice D is incorrect because it wouldn’t make sense to say that recognizing Bosch’s influence on Banisadr isn’t “appropriate,” or suitable. The text indicates that Banisadr himself notes that Bosch’s work has had an effect on him.