Answer A seems like the best choice for this question. Just think about some of the synonyms for therefore—”hence,” “consequently,” and “thus.” Because the adverb therefore often plays the role of a conjunction, it's sometimes called a conjunctive adverb.
B isn’t the correct answer to this question because you use “On the other hand” to introduce two contrasting points, facts, or ways of looking at something.
C isn’t the correct answer because we are not comparing or contrasting anything in this sentence.
D isn’t the correct answer because “Especially” is used in a formal sentence example: She can't be sure she will win, especially at this early stage of the campaign.
