What could cause a shift in the solubility equilibrium?

The correct response to the question regarding what could cause a shift in the solubility equilibrium is indeed related to adding more solute in an unsaturated solution.

In the context of solubility equilibrium, water (or another solvent) can dissolve a certain amount of solute until it reaches saturation—the point at which the rate of dissolving solute equals the rate of solute coming out of solution. When an unsaturated solution is present, it means that there is still capacity for more solute to dissolve without reaching that saturation point. By adding more solute, you are increasing the concentration of the solute in the solution. As a result, the system will respond accordingly, leading to a shift in the solubility equilibrium as it adjusts to accommodate the new amount of solute. This is consistent with Le Chatelier's Principle, which states that a system at equilibrium will shift in response to changes in concentration, temperature, or pressure to counteract the imposed change.

In contrast, the other options do not significantly impact solubility equilibrium in the same way. Doubling the volume of solvent would dilute the solute but wouldn't shift the equilibrium, as the system is still at a lower concentration. Decreasing the temperature in a saturated solution