A Correct. “Teenager” is used as collective noun, so it is mandatory that it is used with a third person plural pronoun.
B Correct. The object pronoun “them” is used as a gender neutral option, even though “teenager” is a single noun.
C Incorrect. The best combination would be with pronouns “him” or “her” because the noun is singular.
D Incorrect. “Teenager” is a single noun, so it should be used with the possessive adjective “its”.
E Incorrect. Because the noun is plural, it should be combined with the pronoun “they”.