Many elements are found in living organisms either free or in the form of compounds. One of the following is not found in living organisms.

Options

A. Silicon
B. Magnesium
C. Iron
D. Sodium

Explanation

To determine which element is not found in living organisms, we can analyze the presence of various elements in human beings and other organisms. Here's a step-by-step solution:

1. **Identify Common Elements in Living Organisms**:
- Living organisms are primarily composed of elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Additionally, there are trace elements like sodium, potassium, calcium, iron, and magnesium that play vital roles in biological processes.

2. **Examine the Elements Mentioned**:
- **Sodium (Na)**: Sodium is commonly found in living organisms, especially in the form of Na+ ions, which are crucial for nerve function and fluid balance.
- **Iron (Fe)**: Iron is essential for many biological functions, including oxygen transport in hemoglobin. It is present in the form of Fe²⁺ and Fe³⁺ ions.
- **Magnesium (Mg)**: Magnesium is another important element, required for various enzymatic reactions and is present in a small percentage by weight in organisms.
- **Silicon (Si)**: Silicon is not commonly found in significant amounts in living organisms. While it may be present in trace amounts in some organisms, it does not play a crucial role in biological processes.

3. **Conclusion**:
- Among the elements listed, silicon is the one that is not found in living organisms in any significant form or function. Therefore, the answer to the question is silicon.

**Final Answer**: Silicon is not found in living organisms.

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