A solution in which more solute can be dissolved is called a(n) __________ solution.

An unsaturated solution can dissolve more solute. 

In general, we can classify solutions as saturated, unsaturated and supersaturated. When the solute has reached its solubility limit, for a given solvent and conditions, the solution becomes saturated. A saturated solution will not dissolve any more solute and any extra solute added will simply stay as it is, without dissolving. An unsaturated solution has less solute than its solubility and hence can still add more solute, till...

An unsaturated solution can dissolve more solute. 

In general, we can classify solutions as saturated, unsaturated and supersaturated. When the solute has reached its solubility limit, for a given solvent and conditions, the solution becomes saturated. A saturated solution will not dissolve any more solute and any extra solute added will simply stay as it is, without dissolving. An unsaturated solution has less solute than its solubility and hence can still add more solute, till it reaches the saturation level. A supersaturated solution has more solute than the solubility limit and hence the extra solute stays in the solvent. Hence, the answer is unsaturated solution. 

Interestingly, solubility is a function of temperature. For example, if we start adding sugar to a glass of water progressively, we will reach saturation, beyond which any extra sugar will only stay put in the solution. However, if we heat the water sample, part or all of the undissolved sugar may dissolve in water. Hence, heat can make a supersaturated solution, saturated or unsaturated.

Hope this helps.