The answer is a) solute.
When a substance is dissolved in a solvent, it is referred to as a solute. In this scenario, carbon dioxide (CO2) is the gas that is being dissolved in water, which acts as the solvent. The result of this process is a solution of carbonated water, where the carbon dioxide gas enhances the properties of the water but does not change its fundamental nature. Understanding the roles of solute, solution, and solvent is key in chemistry, as each component plays a vital role in various chemical processes.