Final answer:
The family member being referred to in the sentence is a cousin, who is the daughter of the grandparents.
Step-by-step explanation:
The correct completion for the sentence is ma cousine.
'La fille de mes grands-parents' means 'the daughter of my grandparents' in English. So, the family member being referred to is a cousin, who is the daughter of your grandparents.
For example, if your grandparents have a daughter, she would be your cousin.
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