Final answer:
The correct answer is B. Are, as it is the form of the verb 'to be' that agrees with the plural subject 'Sara, Mary' in the sentence provided by the student.
Step-by-step explanation:
The correct form of the verb to complete the conversation is B. Are. The question "Sara, Mary, what ______________ you interested in?" requires a verb that agrees with a plural subject. In this case, the plural subject is 'Sara, Mary'. Therefore, the correct sentence would be "Sara, Mary, what are you interested in?". This follows the sentence pattern of (subject) + (be verb) + (adverbial of time or place/adjectival/noun phrase), where 'are' is the 'be verb' that agrees with the plural subject 'Sara, Mary'.