Final answer:
To correct color issues on a CRT monitor as opposed to an LCD monitor, the user should use the Degauss function, which reduces or eliminates remnant magnetic fields that can distort colors (option D).
Step-by-step explanation:
If a user has an LCD and a CRT monitor attached to their workstation and the colors are slightly off on the CRT but not the LCD, the control that should be used in troubleshooting is D. Degauss. The term 'degauss' refers to the process of decreasing or eliminating a remnant magnetic field, which is relevant for CRT monitors. CRT (Cathode Ray Tube) displays can be affected by magnetic fields, which can distort the image and affect color purity. Degaussing can help restore the proper color balance by neutralizing these magnetic effects. In contrast, LCD (Liquid Crystal Display) monitors do not require degaussing because they do not use electron beams influenced by magnetic fields, as CRTs do.
Hence, the answer is option D.