After filling the edges appear darker than the centre. What is the most likely cause?

Correct answer: Repeated painting along the edges created a double layer

In an even colour fill, repeated passes should not accumulate at the boundary. If the painter repeatedly retouches the edges, a second or thicker pigment layer forms there, increasing the local value and making the edges look darker than the centre. Thus option C is the most likely cause. Excess water in the centre may make it lighter, but paper thickness and a new brush do not directly create a darker double layer at the edges.