In an aqueous solution of potassium chloride the solute is

Potassium chloride

Explanation:

A solution is formed by a solvent and one or more solutes.

The solvent is the species that is in major proportion and usually defines the state of aggregation of the solution, while the solute/s is/are in minor proportion.

Also, water is known as the universal solvent, so in any solution containing water, it is considered as the solvent.

Then, in an aqueous solution of potassium chloride the solute is potassium chloride.