Final answer:
The correct possessive adjective to complete the sentence is "son."
Step-by-step explanation:
The correct possessive adjective to complete the sentence is son.
The sentence should be: Son amie va avec nous au cinéma.
This means His friend is going to the cinema with us. In French, possessive adjectives agree in gender and number with the noun they modify.
Learn more about Possessive adjectives in French