When does a backrun or bloom form in watercolour?

Correct answer: When wetter paint or water enters a partly dry area

A backrun, also called a bloom or cauliflower, develops when extra water or wetter paint flows into an area that has begun to dry. The moisture movement pushes pigment toward an irregular boundary, leaving a pale or dark-edged mark. Even moisture does not create this defect, and dry-on-dry painting normally produces a controlled mark rather than a bloom. Thus option D is correct because it describes the sudden wetness imbalance that causes the effect.