What is the most direct way to limit colour spreading in wet-on-wet technique?

Correct answer: Reduce excess surface moisture

The governing concept is controlling moisture in wet-on-wet painting. Colour spreads according to how much water is available on the paper; removing excess surface moisture with a clean, damp-controlled brush or by waiting briefly gives the pigment less freedom to travel. Adding water increases spreading, scrubbing damages fibres, and thin paper is not the direct control. Therefore option D is correct because it addresses the cause of uncontrolled movement.