The correct sentence is "The two teenagers, Saul and she, usually have dinner ready when their grandmother gets home from work."
Nominative pronouns function as the subject of the sentence. This is why they are also known as subjective pronouns. Common nominative pronouns are I, you (both singular and plural), he, she, it, we, they and who.
Objective pronouns are used when something is being done to the subject. Common objective pronouns are me, you (both singular and plural), him, her, it, us, them and whom.
In the given sentence, the correct pronoun is "she". It is a subjective pronoun as it lies in the subject part of the sentence.
The subjects of the sentence are the two teenagers. Since the pronoun refers to one of the two subjects, it must be nominative. Of the given options, "she" is the nominative pronoun. Hence, it is correct.
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