Correct Answer: (c) young teenagers
Solution:'Young' is the general word for something which is undeveloped, immature, and in process of growth and 'infants' is a term used to denote a newborn. Moreover, there is a plural verb 'are' after the relative pronoun 'who' which is used for the antecedent 'infant teenager' but as we know 'teenager' is not plural, we need to change it to its plural form i.e. 'Teenagers'. Hence, 'young teenagers' is the most appropriate answer.