C is the correct answer. For nonmetal-nonmetal compounds, use the Greek prefixes
Mono - 1
Di - 2
Tri - 3
Tetra - 4
Penta - 5
Hexa - 6
Hepta - 7
And so on...
The reason D is not right is because a and o cannot be next to each other in between prefix and element.
Hope this helps!
