What is the main cause of acrylic paint seeping under masking tape?

Correct answer: The tape edge was loose or the paint was too thin

The governing concept is creating a sealed boundary while controlling paint consistency. If the tape edge is not pressed firmly, a gap remains; watery or overly thin paint can then travel beneath it by capillary movement. Option C combines both practical causes. The tape being straight, the purity of the colour, or thick paper does not normally create leakage. Press the tape down and use a controlled paint load to keep the edge clean.