What should be done to keep acrylic letters equal in height?

Correct answer: Draw light baseline and height guidelines

The governing concept is alignment through preliminary guides. A light baseline and one or more height guidelines give every letter a common starting level and upper limit, so option A is correct. Different brushes do not guarantee equal height, a wet background affects drying rather than measurement, and excess water can make strokes run; none of those controls letter height as reliably.