The correct answer is A. Many adults.
Explanation
In the fragment, the words "Many adults" are an antecedent, that is, a previous noun that is replaced by a pronoun (for example he, she, it, we, they) in the following sentence. In the text the author mentioned "Many adults feel ..." and then he said, "They believe...". This means, the noun "many adults" is replaced by the pronoun "they". According to the above, the correct answer is A.
