In order to answer the question, we must first turn to the types of Presumably, most parents (27) _________ are always worrying about their children's safety buy mobile phones for them to track their whereabouts.

Đáp án B

Kiến thức: đại từ quan hệ 

• “parents” là danh từ chỉ người → chọn “who”

Presumably, most parents (27) _________ are always worrying about their children's safety buy mobile phones for them to track their whereabouts.