Final answer:
The correct possessive adjective to complete the sentence is 'leur' which means 'their' in French.
Step-by-step explanation:
The correct possessive adjective to complete the sentence is leur.
The sentence would then read: Ils sont avec leur oncle.
The word 'leur' is a possessive adjective that means 'their' in French and agrees in number and gender with the noun it modifies. In this case, it modifies the masculine singular noun 'oncle.'
Learn more about Possessive adjectives in French