How can an overly thin acrylic mixture be corrected?

Correct answer: By adding a little more base paint

The governing concept is adjustment of paint consistency and covering power. If an acrylic mixture contains too much liquid and has become overly thin, adding a small amount of the original or base paint increases the pigment concentration and restores a workable body. Therefore option B is correct. More water would make it thinner, changing the brush cannot alter the mixture, and adding paper is irrelevant. The base paint should be added gradually and mixed thoroughly.