The correct answer is a) solute.
In the context of solutions, when a substance is dissolved in another, the substance that gets dissolved is referred to as the solute. The substance that does the dissolving is called the solvent. Together, the solute and solvent form a solution. For example, when you mix salt (the solute) into water (the solvent), the result is a saltwater solution. The term ‘dissolving agent’ is not commonly used in scientific discussions, so it is better to stick with the terms solute, solvent, and solution for clarity.