To dilute a solution you either have to solvent or the amount of solute.
answer choices:
add, add to
reduce, add to
add, reduce
reduce, do nothing to

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.

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