Final answer:
The correct plural form is 'Mis hermanas son bonitas'.
Step-by-step explanation:
The original sentence 'Mi hermana es bonita' is written in singular form. In order to change it to the correct plural form, we need to change the possessive pronoun 'Mi' to a plural form such as 'Mis', and the noun 'hermana' to 'hermanas'. So, the correct answer is option a. Mis hermanas son bonitas.
Learn more about Plural forms in Spanish