The best answer is d - solvent
A solvent is a substance, usually a liquid, in which other materials dissolve to form as solution.
In this case, water is the solvent while salt is called the solute, the substance being dissolved. When this happens a solution is formed.
Usually the solvent is the substance that makes up majority if the solution, while the solute is the substance that makes up the minority of the solution.
Salt dissolved in water forms a solution called brine.