Let us clear up the confusion and get a definitive answer: This is that the three quoted colours are defined as the PRIMARY COLOURS: The explanation- definition- is as follows:
Primary colors are colours that can be combined to make a useful range of colors. For human applications, for additive combination of colours, as in overlapping projected lights , your television set, as examples, the primary colors are RED, GREEN and BLUE. (For interest, the yellow colour you see on your television screen is produced by a combination of green and red light).
For subtractive combination of colours, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow.
(There is no combination Red Green and Yellow)
Therefore you fill in your answer as PRIMARY in big capital letters.