The correct answer is A. My sisters
Step-by-step explanation:
According to grammar rules, the subject and verb should always agree on the number, this means a singular subject can only be followed by a verb that is in a singular form and in the same way a plural subject needs to be followed by a plural verb form.
This means, in the sentence "_______ wins every dance contest" the missing subject should be singular as the present verb "wins" can only be used with the singular nouns she, he, it; therefore, all her sister-in-law, the step-it-up sister's captain, and my sister agree with this sentence, but not "my sisters" as this is plural and therefore should be followed by the verb "win" rather than "wins".