"Ozone hole" refers to:

Options

A. Increased concentration of `O_(3)`
B. Reduction in the thickness of ozone layer in the stratosphere
C. Reduction in the thickness of ozone layer in the troposphere
D. Hole in ozone layers.

Answer

To answer the question "Ozone hole" refers to, we will analyze each option provided and determine which one accurately describes the phenomenon.

### Step-by-Step Solution:

1. **Understanding the Ozone Layer**:
- The ozone layer is a region of Earth's stratosphere that contains a high concentration of ozone (O3) and protects the planet from harmful ultraviolet (UV) radiation from the sun.

2. **Definition of Ozone Hole**:
- The term "ozone hole" specifically refers to the depletion or thinning of the ozone layer, particularly over the polar regions, especially Antarctica.

3. **Analyzing the Options**:
- **Option 1**: Increased concentration of O3.
- This is incorrect. The ozone hole is characterized by a decrease in ozone concentration, not an increase.

- **Option 2**: Reduction in the thickness of the ozone layer in the stratosphere.
- This is correct. The ozone hole indicates a significant reduction in the thickness of the ozone layer in the stratosphere due to the presence of ozone-depleting substances.

- **Option 3**: Reduction in the thickness of the ozone layer in the troposphere.
- This is incorrect. The ozone hole specifically refers to the stratosphere, not the troposphere.

- **Option 4**: Hole in the ozone layers.
- This is misleading. While it is referred to as a "hole," it does not mean there is a literal hole; rather, it indicates a thinning of the ozone layer.

4. **Conclusion**:
- Based on the analysis, the correct answer is **Option 2**: "Reduction in the thickness of the ozone layer in the stratosphere."