Answer :
The sentence with proper parallel structure is "We posted the notice wherever people meet, on the Internet, and in the office."
The sentence that contains a proper parallel structure is a. The proposal affects three groups: students, parents, and teachers.
A parallel structure, also known as parallelism, consists of the repetition of a pattern in the sentence. In this case, the only sentence that maintains the repetition of the pattern is the first one since it provides the same importance to students, parents, and teachers. The use of parallel structure is aimed to give equal importance to several elements within the sentence to make it balanced.