"My grandmother was born on a small island."
Translation:Ma grand-mère est née sur une petite île.
I was just puzzling over this (even went to look up île in the dictionary) and realised that when I thought île was masculine before, it was because the exercise had "son île" in it. I was forgetting the rule that if a noun starts with a vowel or mute h, you use the masculine son instead of sa. (Just like previous examples we've had of mon amie.)