Which of the following structures is not found in a prokaryotic cell?

Options

A. Ribosome
B. Mesosome
C. Plasma membrane
D. Nuclear envelope

Explanation

To determine which structure is not found in a prokaryotic cell, we will analyze the options provided:

1. **Ribosome**: Ribosomes are present in prokaryotic cells. They are essential for protein synthesis and are found freely floating in the cytoplasm.

2. **Mesosome**: Mesosomes are invaginations of the plasma membrane in prokaryotic cells. They are associated with cellular processes such as respiration and cell division. Therefore, mesosomes are found in prokaryotic cells.

3. **Plasma Membrane**: The plasma membrane is a fundamental structure in all cells, including prokaryotic cells. It serves as a barrier that regulates the entry and exit of substances.

4. **Nuclear Envelope**: Prokaryotic cells do not have a true nucleus; instead, they have a nucleoid region where their genetic material (DNA) is located. There is no nuclear envelope surrounding this region, which is a key distinction between prokaryotic and eukaryotic cells.

Based on the analysis of the options, the structure that is not found in a prokaryotic cell is the **nuclear envelope**.

### Final Answer:
The structure that is not found in a prokaryotic cell is the **nuclear envelope**.

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