The following element is not found in living organisms either free or in the form of compounds :

Options

A. sodium
B. zinc
C. iron
D. silicon

Explanation

To determine which element is not found in living organisms, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Identify the Elements**: The elements given in the question are sodium, zinc, iron, and silicon.

2. **Evaluate Sodium**:
- Sodium is an essential element in living organisms.
- It plays a crucial role in regulating nerve impulses and maintaining membrane permeability.
- Therefore, sodium is found in living organisms.

3. **Evaluate Zinc**:
- Zinc is another essential trace element found in living organisms.
- It is vital for the immune system and is present in various cellular functions.
- Thus, zinc is also found in living organisms.

4. **Evaluate Iron**:
- Iron is a critical element for many biological processes, particularly in blood production.
- Approximately 70% of the body's iron is found in red blood cells (RBCs), making it essential for oxygen transport.
- Hence, iron is found in living organisms.

5. **Evaluate Silicon**:
- Silicon is not typically found in living organisms.
- Its presence in biological systems is negligible, and it does not play a significant role in biological functions.
- Therefore, silicon is the element that is not found in living organisms.

### Conclusion:
The element that is not found in living organisms, either free or in the form of compounds, is **silicon**.

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