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.
<span>Thiou is a small river 3.5km <span>long. Le "Thiou" est une petite rivières de 3,5 km de long.(déversoir du lac d'Annecy en Savoie) La Manche "channel river" </span></span> <span>The Rhine is a French <span>river (le Rhin est un fleuve français)</span></span>
