What is characteristic of a white dwarf?

A white dwarf is a stellar remnant that is primarily composed of carbon and oxygen and forms when a low to medium mass star exhausts its nuclear fuel and expels its outer layers. After this expulsion, what remains is the hot core of the star which no longer undergoes nuclear fusion and has a very small radius but a mass similar to that of the Sun.

The choice of slow consumption of depleted hydrogen fuel is correct because, after a star has become a white dwarf, it has already completed its hydrogen fusion phase. The core, now a white dwarf, is composed mostly of carbon and oxygen, and it will gradually cool and fade over billions of years. During this cooling process, some white dwarfs may slowly convert any remaining helium that has been produced in previous helium fusion phases, but there is no ongoing hydrogen fusion as the hydrogen fuel supply has been depleted.

The other choices represent processes or characteristics not associated with white dwarfs. Rapid expansion is characteristic of other phases like a supernova or red giant phase, formation of a black hole involves massive stars rather than those that become white dwarfs, and immediate helium fusion occurs in newer stars that are in the main sequence phase rather than in the white dwarf stage where fusion has already ceased.