Answer: d. Compounds made by bonding of atoms of known elements.
Step-by-step explanation:
Viruses are said to be so numerous that they vastly outnumber the number of stars in the universe. Viruses are quite small and are not technically alive until they get into a living cell.
Viruses are made up of compounds like everything else but just because a virus has not been discovered does not mean that it is made of unknown material. Every natural element available has already been discovered so any new virus is made of compounds of known elements.