Answer: D.) Nucleus
Explanation: In the question, we are told to find a structure that is found ONLY in eukaryotic cells. Because of this, we have to eliminate any option that is found only in prokaryotic cells, or any that is found in both prokaryotic and eukaryotic cells. In options A-C, we are given structures found in both types of cells. The cytoplasm, DNA, and ribosomes differ in each type of cell; however, nevertheless, they do not only appear in eukaryotic cells. This makes nucleus the only valid option, as it is only found in eukaryotic cells.
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