Final answer:
The correct conjugation is 'son'.
Step-by-step explanation:
The correct conjugation in this sentence is 'son'.
Mis hermanas is a plural subject, so we need a plural verb form. 'Son' is the correct conjugation of the verb 'ser' (to be) for the third person plural. This sentence translates to 'My sisters are very beautiful.'
Learn more about conjugation in Spanish