Explanation

In the sentence "He is poor but honest," the word "but" functions as a conjunction. Conjunctions are words that connect clauses, phrases, or words within a sentence. In this case, "but" is used to link two contrasting ideas: the fact that he is "poor" and the fact that he is "honest."

The use of "but" suggests a relationship where the second clause provides a contrast to the first. Here, despite his poverty, he possesses the positive quality of honesty. This juxtaposition emphasizes the remarkable nature of his honesty in light of his financial status.

Looking at the other options: "poor" and "honest" are adjectives describing the subject "he," while "he" is a pronoun serving as the subject of the sentence. None of these words function to connect ideas within the sentence as "but" does. Therefore, the correct answer is indeed C) but, as it is the conjunction that unites these two contrasting characteristics of the subject.