The correct answer is: D. having each electron gun controlled by separate signal.
In raster-scan color monitors, each electron gun is responsible for one of the three primary colors (red, green, and blue). By controlling the intensity of each electron gun, the monitor can create a wide range of colors.
Option A is incorrect because grouping the phosphor dots in a circular pattern does not affect the intensity of the colors.
Option B is incorrect because having each electron gun controlled by a composite signal does not allow for independent control of each color.
Option C is incorrect because using an in-line gun arrangement with a solid metal mesh does not allow for independent control of each color.
Option E is incorrect because having each electron gun controlled by a separate signal is the only way to achieve independent control of each color.