Assertion : PCR primers must not have self complementary regions.
Reason : Self complementary regions result in hairpin structures adversely affecting the PCR.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "PCR primers must not have self-complementary regions." This means that the sequences of the primers should not be able to bind to themselves.

2. **Evaluating the Reason**: The reason given is that "self-complementary regions result in hairpin structures adversely affecting the PCR." This indicates that if a primer has regions that can bind to themselves, they will form hairpin structures.

3. **Explaining Hairpin Structures**: A hairpin structure occurs when a single strand of DNA or RNA folds back on itself and forms base pairs with itself. This can prevent the primer from binding to the target DNA during PCR, making it ineffective.

4. **Conclusion on the Assertion and Reason**: Both the assertion and the reason are true. The assertion is correct because PCR primers should not have self-complementary regions to function properly. The reason is also correct because self-complementary regions lead to hairpin structures, which hinder the PCR process.

5. **Choosing the Correct Option**: Given that both the assertion and the reason are true, and the reason correctly explains the assertion, the correct option is:
- **Option A**: Both assertion and reason are true, and the reason is the correct explanation for the assertion.