Of the diodes shown in the following diagrams, which one is reverse biased?

Options

A.
B.
C.
D.

Explanation

To determine which diode is reverse biased from the given diagrams, we need to understand the conditions for reverse bias in a PN junction diode.

### Step-by-Step Solution:

1. **Understand the Diode Structure**:
- A diode consists of two regions: P-type (positive) and N-type (negative).
- The P-type region has an abundance of holes (positive charge carriers), while the N-type region has an abundance of electrons (negative charge carriers).

2. **Identify Reverse Bias Condition**:
- A diode is reverse biased when the P-type side is connected to the negative terminal of the battery and the N-type side is connected to the positive terminal.
- This configuration prevents current from flowing through the diode.

3. **Analyze the Given Diagrams**:
- For each diode in the diagrams, check the connections:
- If the P-side is connected to a lower potential (negative terminal) and the N-side is connected to a higher potential (positive terminal), then that diode is reverse biased.

4. **Evaluate Each Diagram**:
- **Diagram 1**: P-side connected to -12V and N-side connected to -5V.
- Here, the P-side is at a lower potential than the N-side, so this is forward biased.
- **Diagram 2**: P-side connected to 0V and N-side connected to -10V.
- The P-side is at a higher potential than the N-side, so this is also forward biased.
- **Diagram 3**: P-side connected to 5V and N-side connected to 0V.
- Again, the P-side is at a higher potential than the N-side, indicating forward bias.
- **Diagram 4**: P-side connected to -10V and N-side connected to 5V.
- In this case, the P-side is at a lower potential than the N-side, indicating reverse bias.

5. **Conclusion**:
- The diode that is reverse biased is the one where the P-side is connected to -10V and the N-side is connected to 5V.

### Final Answer:
The diode in Diagram 4 is reverse biased.