In prokaryotic cells, the nucleus is absent, and their DNA floats in the cytoplasm. Eukaryotic cells, on the other hand, contain a defined nucleus that holds their DNA. Thus, the correct answer is option C: nucleus.
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In prokaryotic cells, the nucleus is not present as it is in eukaryotic cells. Instead, prokaryotic cells have their DNA free-floating in the cytoplasm. Therefore, the nucleus is the structure found in eukaryotic cells but absent in prokaryotic cells. ;