The correct answer is: b. nebula.
Explanation:
1. After a supernova explosion, the outer layers of the star are ejected into space, forming a cloud of dust and gas called a nebula.
2. This nebula can serve as the birthplace for new stars and planets as gravity causes the material to come together and form new celestial bodies.
3. While some supernovae can result in the formation of neutron stars or black holes, in this case, 90 percent of the star's mass becoming a nebula refers to the dispersal of material into space rather than the formation of a compact stellar remnant like a neutron star or black hole.
Therefore, the most fitting option in this context is b. nebula.
