Answer:
The answer is indeed letter d. unparallel structure.
Explanation:
Parallelism consists of the repetition of a certain grammatical structure inside a sentence to avoid confusion by creating consistency. We must use it when we wish to convey more than one idea in a single sentence. It makes the speech clear, interesting, and easy to understand.
In the sentence "The singer not only was talented but she was a nice person, too", we can notice a lack of parallelism. It would sound clearer if the grammatical forms following the correlative conjunction "not only... but also..." were the same. Let's compare the original sentence with the corrected one:
The singer not only was talented but she was a nice person, too.
The singer was not only talented but also nice.
By placing the verb "was" before "not only", we make the sentence more direct, since there will be no need to repeat the verb later. Then, all we have after "not only" is an adjective, "talented". Thus, all we need to place after "but also" is as adjective as well, "nice".
Note that we chose to use to conjunction as it should be, "but also", instead of substituting "also" for "too" - that substitution only prevents parallelism.
