The answer is A. - Prokatyotic cells lack a defined nucleus while eukaryotic cells do have one :) I loved learning about the cells in biology! But I noticed your question is worded a little differently so the nucleus would be the one thing a eukaryotic cell has that a prokatyotic cell does not...
If the question is, which feature is found in eukaryotic cells but not in prokaryotic cells, then the answer is A. nucleus ( as prokaryotes lack a well defined nucleus).
