A light detail over a dark poster colour area keeps becoming too thick. What is the best control method?

Correct answer: Use limited thick paint in a fine pointed brush and make one decisive stroke

The governing concept is controlled brush technique. A fine pointed brush gives a narrow, predictable mark, and a limited amount of suitably thick paint prevents spreading. One decisive stroke avoids piling up repeated layers, so option A is correct. A broad brush makes the detail wider, excess water can weaken coverage and cause spreading, and repeating the line necessarily increases its thickness.