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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when you are finished using a light microscope or any microscope in general you always need to put the cover back on it. Mostly, to protect it from any harmful bacteria, light etc. and to keep it clean and, from collecting dust.
