The right answer is water.
A solution is a liquid containing several constituents
* The majority constituent is called solvent. Most often consists of water, the solution is called aqueous solution.
* The substances in solution are called solutes.
* The species in solution are either molecules or ions.
If we add water in the aqueous solution, we will have a dilution. On the contrary, if we add solute or remove water (pure water in solutes) we will have a concentration.