What solution is formed when sand is added to water?

A solution has two components solvent and solute. Solute is in small quantity and solvent is in large quantity. When the solute is fully miscible in solvent then we call it a solution. For instance, Salt when added in water dissolves completly, so we call it "Salt solution".
A mixture contains two or more substances which are not chemically combined. Components of mixture can be easily seperated using physical methods like filteration, simple distillation, fractional distillation, freezing, heating, etc. Examples of common mixtures are: Sea water, Ink, Crude oil, Gun powder, etc.
When we add sand into water, sand is not miscible in water and sand settles down at the bottom of the container. We can easily separate sand from water by physical methods like filteration, drying and obtain the same sand which we add earlier.

A solution is formed when a solute is dissolved into a solvent. An example is the sweet solution that is formed when sugar is mixed with water or the salty solution that is formed when common salt is dissolved into water.
In comparison, when sand is added to water, it does not mix, and no solution is formed. Because the sand is heavier than the water, it will ultimately settle down at the bottom of the container. The resulting product is, in fact, a mixture.
The mixture of sand and water can be separated out by physical actions of filtration and drying. Water containing sand can be filtered to separate out sand and then dried to obtain dry sand (the original material we started off with).
Hope this helps.