The correct option is D Adverb
An 'adverb' tells us the way someone does something, or in other words how a verb is performed by modifying the adjective, verb or other adverbs; while an adjective describes the verb itself. In this sentence, 'therefore' is providing the reasoning for the verb (is) and is also connecting the two halves of the sentence, and therefore is a conjunctional adverb. Thus, Option D) is correct.