The correct option is A by adding more solute to the solvent
Unsaturated solutions have the ability to dissolve more solute in them untill they reach the saturation point. After reaching the saturation point, solutes will no more get dissolved in the solvent and these are the unsaturated solutions. Hence all the solutions are said to be mostly unsaturated in nature and finally get converted to a saturated solution by adding solute in them.
Hence option A is correct.