C. 78% of the earth's atmosphere (8)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
nitrogen
(Other definitions for nitrogen that I've seen before include "Neon grit (anag.)" , "that's dominant in the air" , "Inert nog is an inert gas" , "Major constituent of air" , "Constituent of four fifths of earth's atmosphere" .)