It is (but) (=only) right to admit our faults.

Explanation

In this sentence, "but" functions as an adverb, emphasizing the exclusivity of the statement and serving as a synonym for "only."Why the other options are wrong:(B) Noun: "But" is not functioning as a noun in this context.(C) Interjection: "But" is not expressing strong emotion independently.(D) Pronoun: "But" is not replacing a noun; it is an adverb.