Answer: The broadest group for classification is "Domain".
Domain is a taxonomic group that lies above kingdom level and divides all life forms into three broad categories that is Archea, Bacteria, and Eukarya.
On the contrary, Taxon is a rang or a unit (like species, genus, class) that is designated in the biological classification of organisms.
Thus, b) Domain is the right answer.
1. Aurora
2. Ion
3. Thermosphere
4. Troposphere
5. Stratosphere
6. Ionosphere
7. Mesosphere
8. Ozone
9. Ozonosphere