Final answer:
The pronoun 'ellas' can be used to replace the subject in the given Spanish sentence.
Explanation:
Ellas can replace the subject in the sentence 'Mis sobrinas se llaman Marta y María.'
In Spanish, the pronoun tú is informal singular, usted is formal singular, and yo is used for 'I.'
Understanding pronouns is crucial in language as they replace nouns and help in creating coherent sentences.
Learn more about Pronouns in Spanish
