The action after which the concentration of a solution would remain constant is:
adding solute to the solution.
When you add solute to a solution, the concentration stays constant if the amount of solvent stays the same or if you add enough solvent to maintain the same ratio of solute to solvent. In general, the only action from the provided options that wouldn't change the concentration would be adding an appropriate amount of solute that keeps the concentration unchanged relative to the existing volume.
However, if none of the options provided perfectly describe a scenario where concentration truly remains constant (like adding exactly the same amount of solute as the amount already present), you might consider this response as a more context-dependent choice. In general terms, adding solute increases concentration unless balanced precisely.
None of the other listed actions would result in a constant concentration; adding water would dilute the solution, evaporating water would concentrate the solution, and removing solution would increase concentration.