Answer: a. Yellow
According to the additive theory of color, when we join the three primary colors of light (Red + Green + Blue) we get White light.
On the other hand we have secondary colors of light that are:
Yellow = Red + Green
Magenta = Blue + Red
Cyan = Blue + Green
Now, if we know that:
Red + Green + Blue = White
And:
Red + Green = Yellow
Then:
Yellow + Blue = White
