What would you take to an uninhabited island? That's a common question of curiosity (is there a better term for this kind of questions?). As the question usually implies the means of survival in such an environment a satellite phone would be an obvious answer. Your best chance to survive would be getting out of there as soon as possible. The phone can both call you a ride and provide your exact location.
Of course the question can be approached in more ways. The answer would be different if it was implied you are there for a reason. For example a challenge to survive for a set amount of time. Means to get and prepare food, shelter and above all fresh water should be at the top of the list. But most of those can be achieved with some basic survival skills. Antibiotics should be the one thing you should bring if that was the situation. Everything else is attainable on site but it's highly probably you'll get sick from exhaustion, bruises, small cuts and all the odd things you have to eat.
Sometimes the survival is granted and the question is intended more on how would you pass time in such situation, or what is the dearest thing you wouldn't like to live without. Unlike the previous scenarios there is no right answer to this version. It's more personal and the right one of you would like to learn more about the person instead of evaluating their reasoning.