Answer:
The correct answer is letter A. They
Step-by-step explanation:
Despite "They" and "Them" are pronouns, the main difference between these two pronouns is that the first one you use like a subject pronoun and the second one as an object pronoun. In this case, if you want to use "them" ina sentence, you have to use it after a verb or a preposition, because the reason it works as an object pronoun. In the sentence of this question, to use "Them", you would have changed it like the following sentence: Though Ed and his sisters want to study whales, Ed is more serious about them. Even though this sentence doesn't make any sense, it's such an example about the using of "Them".