It is estimated that mainly ..... of thee earth's terrestrial surface is covered by lichens .

Options

A. `18%`
B. `12%`
C. `8%`
D. `4%`

Answer

To solve the question regarding the percentage of the Earth's terrestrial surface covered by lichens, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the estimated percentage of the Earth's terrestrial surface that is covered by lichens.

2. **Define Terrestrial Surface**: The terrestrial surface refers to the land areas of the Earth, excluding water bodies.

3. **Identify What Lichens Are**: Lichens are a symbiotic association between fungi and algae. The algal component is called the phycobiont, and the fungal component is called the mycobiont.

4. **Research the Coverage of Lichens**: It is known that lichens cover a significant portion of the Earth's terrestrial surface. Research indicates that lichens cover approximately 8 to 10% of this surface.

5. **Evaluate the Options**: The options provided are:
- Option 1: 18%
- Option 2: 12%
- Option 3: 8%
- Option 4: 4%

6. **Analyze the Options**:
- 18% and 12% are too high compared to the known estimate of 8 to 10%.
- 4% is too low.
- Therefore, the only reasonable option that fits the estimate is 8%.

7. **Conclusion**: The correct answer is that approximately 8% of the Earth's terrestrial surface is covered by lichens.

### Final Answer:
**Approximately 8% of the Earth's terrestrial surface is covered by lichens.**