Which observation proves that a cell is a eukaryote?

The correct answer is D. It has DNA inside a nucleus.

Eukaryotes are cells that have a nucleus, which is a membrane-bound organelle that contains the cell’s DNA. Prokaryotes are cells that do not have a nucleus. They have their DNA in a region of the cell called the nucleoid.

Option A is incorrect because all cells contain DNA.

Option B is incorrect because ribosomes are found in both prokaryotic and eukaryotic cells.

Option C is incorrect because a cell membrane is a feature of both prokaryotic and eukaryotic cells.

Therefore, the correct answer is D. It has DNA inside a nucleus.