In a dark-to-light gradation the middle became suddenly too light. Which cause is most appropriate?

Correct answer: Too much water was added in one step

A smooth dark-to-light gradation requires the pigment-to-water ratio to change gradually. When too much water is added in one step, the pigment becomes suddenly diluted, so the middle loses value and appears much lighter than the surrounding sections. Thus option D is correct. A small brush mainly affects coverage, white paper is expected in a transparent wash, and excess dark colour would make an area darker rather than suddenly pale.