The correct answer is a.: specific.
This structure (this +adj) requires an adjective and the only adjective is in a., the other options are verbs or nouns and are not correct.
That sentence is an example of parallel structure.
When there is repetition of a specific grammatical form in a sentence, parallelism or a parallel structure is produced. This occurs when you mantain the same pattern for every compared word or idea in your construction. This makes the idea easier to follow by the reader.
