The largest reservoir of nitrogen on earth is :

Options

A. soil
B. oceans
C. air
D. granite rocks

Answer

To solve the question "The largest reservoir of nitrogen on earth is:", we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Options**: The options given are soil, ocean, air, and granite rock.

2. **Understand Nitrogen Distribution**: Recognize that nitrogen is a crucial element for living organisms and is found in various forms in the environment.

3. **Analyze Each Option**:
- **Soil**: While soil contains nitrogen in the form of organic matter and ammonium, it is not the largest reservoir.
- **Ocean**: Oceans do contain nitrogen, but it is primarily in the form of dissolved compounds and is not as significant as other reservoirs.
- **Air**: The atmosphere is composed of approximately 78% nitrogen gas (N2), making it the largest reservoir of nitrogen on Earth.
- **Granite Rock**: Granite rock contains very little nitrogen compared to the other options.

4. **Conclusion**: Based on the analysis, the air is the largest reservoir of nitrogen on Earth.

5. **Final Answer**: Therefore, the correct answer is **Air**.