Hello!
The correct way to rewrite the following sentence using object pronoun(s) is Las amo
The pronoun, in this case, is Las. Las is firstly a pronoun used for feminine subjects. In this case, the subject is "hermanas" ("sisters" in English). As "sisters" is feminine, the pronoun should be feminine too.
Next, Las is a pronoun used for plural subjects. "Sisters" is a plural subject, so a plural pronoun should be used.
Of the listed pronouns, the only one that is feminine and plural is "Las", meaning that this is the correct object pronoun.
Have a nice day!
