Assertion : At high altitude a person, from plain areas, may experience altitude sickness.
Reason : At high altitude atmospheric pressure is generally high leading to symptons like nausea, fatigue, etc.

Explanation

To solve the assertion-reason question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "At high altitude, a person from plain areas may experience altitude sickness."
- This is true because individuals who are not acclimatized to high altitudes often experience symptoms such as nausea, fatigue, and headaches due to the lower oxygen levels available at such elevations.

2. **Understanding the Reason**:
- The reason states that "At high altitude, atmospheric pressure is generally high leading to symptoms like nausea, fatigue, etc."
- This statement is false. At high altitudes, atmospheric pressure is actually lower than at sea level or in plain areas. The decrease in pressure results in lower oxygen availability, which is what causes altitude sickness.

3. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer to the assertion-reason question is that the assertion is true, but the reason is false.

### Final Answer:
- The assertion is true, and the reason is false. Therefore, the correct option is C.