Which observation proves that a cell is eukaryote

The observation that will prove that a cell is eukaryote is, the presence of genetic material within a well defined nucleus which is separated from the cytoplasm by the double layered nuclear membrane.

Explanation:

Cells can be prokaryotic or Eukaryotic depending on the presence or absence of nucleus in them.

Bacteria, archaebacteria and cyanobacteria are prokaryotic and all other known higher organisms are eukaryotic.

The feature that differentiates a prokaryotic cell from an eukaryotic cell is the absence of a distinct nucleus in them.

Eukaryotes have their genetic material enclosed by a double layered nuclear membrane while prokaryotes have their genetic material laid free in their cytoplasm.

The another difference is the lack of membrane bound organelles in prokaryotes which are present in eukaryotes.

Some eukaryotic cells lose nucleus at maturity, but they have it in their premature stages. For example, Red blood cells of humans.