Posting the answer before I forget abt it.. this is from the mock-test:
This sentence displays a special kind of pronoun-antecedent agreement error. Describing kingdom Eubacteria as one of something means that the entity that the singular kingdom is one of must be plural.
Think about simpler cases: "This program is one of my favorite shows." NOT "This program is one of my favorite show."
Kingdom Eubacteria is the only one of...the classification system (a singular noun) is incorrect.
Additionally, the original construction is unnecessarily wordy and arguably redundant. "Only" and "one" mean almost the same thing; if Eubacteria is "only" or "alone", we know that it is "one".
Choice B retains the "singular item out of a singular entity" error. Additionally, they is used to refer to a singular pronoun, kingdom Eubacteria.
Choice C uses a simpler and grammatically correct construction to express the same idea. Kingdom Eubacteria, a singular noun, agrees with the singular verb displays. Alone among is an idiomatically correct expression. That comprise the classification system is an adjective clause immediately following the noun it describes, the six kingdoms. The correct answer is C.