Pollen grains can be stored for years in liquid nitrogen, maintained at temperature:

Options

A. `-120^(@)C`
B. `-20^(@)C`
C. `-70^(@)C`
D. `-196^(@)C`

Answer

### Step-by-Step Solution:

1. **Understanding Pollen Grain Storage**: Pollen grains are the male gametes of flowering plants and have a limited viability period, which is the time they can remain viable for germination.

2. **Factors Affecting Viability**: The viability of pollen grains depends on various factors, including temperature and humidity. Different plant species have different viability periods.

3. **Cryopreservation Technique**: To extend the viability of pollen grains, a technique called cryopreservation is used. This involves storing pollen at very low temperatures to halt metabolic processes and preserve the grains.

4. **Identifying the Correct Temperature**: The question provides four temperature options for storing pollen grains in liquid nitrogen:
- A) -120°C
- B) -20°C
- C) -70°C
- D) -196°C

5. **Selecting the Right Answer**: Among the given options, the correct temperature for storing pollen grains in liquid nitrogen is -196°C. This extremely low temperature is effective for long-term preservation.

6. **Conclusion**: Therefore, the answer to the question is D) -196°C.