C. cyan m magenta y yellow k black
The CMYK model is aimed at printing media, that is, based on the light absorption/reflection characteristics of ink, the color seen by the eyes is actually the color that the object absorbs light of a specific frequency in white light and reflects the rest light.
Each CMYK four-color ink can use a value from 0 to 100%. The percentage of printing ink color specified for the brightest color is low, while the percentage specified for the darker color is high. For example, bright red may contain 2% cyan, 93% magenta, 90% yellow and 0% black.
The color pickers in PS -RGB (additive color) and CMY (subtractive color) are complementary colors.
RGB plus black as the background color, that is, both RGB are 0 and black, and both RGB are 255 and white.
CMY takes white as the background color, that is, CMY is all white, 100% black (but in practice, because of the purity of ink and other problems, pure black cannot be obtained, so K is introduced).
In PS, CMYK mode should be used when preparing to print images with printing colors. If you start with an RGB image, you'd better edit it first and then convert it to CMYK mode. If you print the picture output in RGB mode directly, the actual color of the printed matter will be very different from the RGB preview color.
Why is the last k of CMYK black instead of the first letter of black? Mainly to distinguish between blue and blue.