Explanation:

- The sentence given is: "My sister is just sixteen and therefore not eligible to vote."

- Therefore is the word in question. Let's figure out which word class it belongs to.

- Option 1, Gerund: A gerund is a verb form that functions as a noun, typically ending in -ing. This is not applicable here.

- Option 2, Adverb: Adverbs modify verbs, adjectives, or other adverbs and can express manner, place, time, or degree. Therefore is used to explain a reason, hence an adverb.

- Option 3, Adjective: Adjectives describe or modify nouns. Therefore does not describe a noun here.

- Option 4, Conjunction: Conjunctions connect words, phrases, or clauses. Therefore doesn’t serve this function here.

- Option 2, Adverb is correct as therefore modifies the clause by linking it to the previous statement, providing a reason.