Answer:
D) hot stars and interstellar gas, particularly hydrogen
Explanation:
Emission nebula is the nebula which is formed of the ionized gases which emit light of different wavelengths. The common source of ionization of the emission nebula is high-energy photons mostly which are emitted from hot star present nearby to it.
The emission nebulae include H II regions. An H II region is the region of the interstellar atomic hydrogen which is ionized. It is the cloud of the partially ionized gas in which star formation takes place.
