As far as I know, sand usually stays near the sea so la mer is the correct option. Also, if you forget, the seaside in French is le bord de mer which has mer (sea) in it.
Explanation: A means the countryside, B means a plain (grassland), C means the mountains and D means the sea so obviously the the sea is beside the sand on beaches which is why you would go there.
The answer is C because in French there is something called le COD and Le COI le COD is the doctor which is replaced by Le but there is no COI in this sentence
