mineral water is a solution due to it having dissolved minerals which you cannot dictate with your eyes,making them soluble in water.
You should remember the definitions of a solution, compound and suspension and how they differ.
Suspension are called all heterogeneous mixtures, that contain solid particles, while solutions are called all homogeneous mixtures, where solid particles are totally dissolved.
Hence, mineral water contains minerals that are completely dissolved, hence, from this point of view, mineral water is a solution.
You should also know why mineral water is considered a solution and not a compound. Notice that you may obtain the components from mineral water using mechanical methods, instead of chemical decomposition reactions, hence, based on this property, you can classify mineral water as solution.