Final answer:
The correct possessive adjective is 'mon'
Step-by-step explanation:
The correct possessive adjective to fill in the blank is mon.
So the complete sentence would be: Ma cousine, c'est la fille de mon oncle.
The possessive adjectives are used to show ownership or possession. In this case, the speaker is referring to their own uncle, so they would use the possessive adjective 'mon' (my) before the noun 'oncle' (uncle).
Learn more about Possessive adjectives in French