12. Jack has a passion for outdoor activities, among _____ cycling is his favorite.
(A)that
(B)which
(C)what
(D)them

among which是引導定語從句的，among them是不引導定語從句的。

用作定語的從句叫做定語從句(attributive clause)。定語從句通常皆置於它所修飾的名詞(或代詞)之後，這種名詞(或代詞)叫做先行詞(antecedent)。引導賓語從句的關聯詞為關係代詞和關係副詞。關係代詞在定語從句中可用作主語、賓語、定語等；關係副詞在定語從句中只用作狀語。如：

The student who answered the question was John. 回答問題的那個學生是約翰。 (who answered the question是關係代詞who引導的定語從句，用以修飾who先行詞student，who在從句中用作主語)

I know the reason why he was so angry. 我知道他這麼生氣的原由。 (why he was so angry是關係副詞引導的定語從句，用以修飾why的先行詞reason，why在從句中用作原因狀語)

-百度“among which” 与“among them”的用法详解及区别？ 星辰明锐