The correct answer is (a). Red, green, and blue are the primary additive colors. When these colors are combined in different proportions, they can create all the other colors that we see. For example, mixing red and green light creates yellow light, and mixing red, green, and blue light creates white light.
Cyan, magenta, and yellow are the primary subtractive colors. When these colors are combined in different proportions, they can absorb all the other colors of light, leaving only the color that is complementary to the colors that are being absorbed. For example, cyan absorbs red light, magenta absorbs green light, and yellow absorbs blue light. When all three colors are combined, they absorb all colors of light, leaving black.
Red, green, and blue are used in color monitors because they are the colors that our eyes are most sensitive to. When these colors are combined in different proportions, they can create a wide range of colors that we can see.