Which of the following sentences shows proper parallel structure? a) judith dislikes people who are rude, selfish, and they are self-centered. b) judith dislikes people who are rude, selfish, and self-centered. c) judith dislikes people who are rude, selfish, and very self-centered. d) judith dislikes the people who are sometimes rude, act selfish, and self-centered.

The sentence that shows proper parallel structure is B) Judith dislikes people who are rude, selfish, and self-centered.

It is called parallel structure to the repetition of a grammatical form within the same sentence. In other words, it refers to the use of the same grammatical pattern in order to make the sentence clearer for the reader.

In sentence B) "people" is modified by a defining relative clause ("who are rude, selfish and self-centered"). The defining relative clause includes three adjectives ("rude", "selfish" and "self-centered") and that is what makes the sentence a parallel structure.

Sentence B) is the only sentence where grammatical forms have not been mixed. Sentence A) presents two independent clauses  ("Judith dislikes people who are rude, selfish" and "they are self-centered"), which results in the separation of the last adjective and a breakdown in the parallel construction. In sentence C) "self-centered" is premodified by the adverb "very",  which also creates a faulty parallelism. Sentence D) does not show a proper parallel structure either since it includes a verb ("act") in the defining relative clause.  

Given that the definition of parallelism is a sentence that has several of the same speech pattern, Option B is the only option that correctly applies parallelism since it’s the only option that describes the three adjectives in the same form, "rude, selfish and self-centered" the other options have an addition to the self-centered part "and, who are rude, they are self-centered" so that is what makes them incorrect at least doesn’t applies to the parallelism rule.