What should be done to increase transparency in watercolour?

Correct answer: Add a controlled amount of water and apply a thin layer

The governing concept is paint transparency: a dilute watercolour wash lets the paper or earlier layer show through. Adding a controlled amount of water and applying a thin, even layer lowers pigment concentration without making the wash uncontrolled, so option A is correct. Clay and a thick layer make the colour opaque, while a completely dry brush does not create a transparent wash.