Parallel structure refers to having at least two divisions within each level of your outline.
the sentence that is not logically connected in the paragraph is 4).
there must be no period after "word" in order to logically connect sentence 4) to sentence 3) by means of the word "but" preceded by a comma ("radio and tv may enhance the spoken word, but they tend to lower comprehension levels"). "but" has to be added in order to connect two ideas that contrast (radio and tv may have a positive impact, but they can also have a negative impact).
the answer is b. hope this !