The right answer is b) nucleus.
Eukaryotes (or Eukaryota) are a domain grouping all unicellular or multicellular organisms that are characterized by the presence of a nucleus and usually mitochondria in their cells. He opposes the domains of Eubacteria and Archaea.
Eukaryotes gather four great kingdoms of the living world: animals, mushrooms, plants and protists (or protozoa)