Concept:The secondary colours of light are formed by mixing two primary colours of light equally.
Explanation:The primary colours of light are red, green and blue.
Mixing two primary colours gives the secondary colours: cyan = green + blue, magenta = red + blue, and yellow = red + green.
Now examine each given set.
Set i has red, blue and green, which are all primary colours, not secondary colours.
Set ii has blue, cyan and magenta; blue is a primary colour, so this set is not secondary colours only.
Set iii has green, magenta and yellow; green is a primary colour, so this set is not secondary colours only.
Set iv has yellow, cyan and magenta, which are the three secondary colours of light.
Thus, only set iv contains secondary colours only.
Answer:C. iv only