Answer:
Add, reduce
Explanation:
Concentration is essentially a ratio of the amount of solute to the amount of solvent.
\text{Concentration} = \dfrac{\text{amount of solute}}{\text{amount of solvent}}
If you want to make a solution more dilute (less concentrated), you must do one of two things:
A is wrong. If you add both more solute and more solvent, the concentration will stay about the same.
B is wrong. If you add more solute and reduce the amount of solvent, the concentration will increase.
D is wrong. If you add more solute and do nothing to the amount of solvent, the concentration will increase.