Bill suddenly realized how to solve a difficult problem. According to Wallas, Bill has just completed the creative process step called

Select one:

a. verification.

b. incubation.

c. preparation.

d. illumination.

Illumination

Explanation:

Illumination is the stage of creative thinking.

There are four stages of creativity

Preparation Incubation Illumination Verification.

Illumination is the third stage of creativity. In the incubation process, suddenly those ideas get faded, will suddenly reappear in the illumination stage. The things that were not clear, get instantly clear in this stage. The instant idea of a solution that comes in mind is called illumination or an aha feeling.

Thus here Bill suddenly gets the solution of the problem is the result of aha feeling of illumination.