A medium-mass star will end its life cycle as a white dwarf. However, since that option isn't listed among your choices, the closest answer would be a black dwarf, which is the theoretical final state of a white dwarf after it has cooled sufficiently.
To clarify the life cycle:
- A medium-mass star (like our Sun) evolves into a red giant.
- It sheds its outer layers, leaving behind a hot core that becomes a white dwarf.
- Over a long period, that white dwarf could eventually cool down to become a black dwarf.
So while a medium-mass star itself doesn't directly become a black dwarf, it does go through a stage of life where it turns into a white dwarf, which could eventually become a black dwarf. Therefore, the best answer from your choices is a black dwarf.