Assertion: Aniline does not undergo Friedel Crafts reaction.
Reason : Friedel Crafts reaction is an electrophilic substitution reaction.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Aniline does not undergo Friedel-Crafts reaction."
- Aniline (C6H5NH2) is an aromatic amine. The presence of the amino group (-NH2) on the benzene ring significantly affects its reactivity.

2. **Understanding Friedel-Crafts Reaction**: The Friedel-Crafts reaction is an electrophilic aromatic substitution reaction that typically involves the introduction of an alkyl or acyl group onto a benzene ring using a Lewis acid catalyst (like AlCl3).
- In this reaction, the benzene ring acts as a nucleophile and reacts with an electrophile.

3. **Effect of the Amino Group**: The amino group in aniline is a strong electron-donating group due to the lone pair of electrons on the nitrogen atom. This electron donation increases the electron density on the benzene ring, making it more reactive towards electrophiles.
- However, the lone pair on nitrogen can coordinate with the Lewis acid (like AlCl3), which prevents the formation of a suitable electrophile for the Friedel-Crafts reaction.

4. **Conclusion about the Assertion**: Since the amino group can coordinate with the Lewis acid, aniline does not effectively participate in the Friedel-Crafts reaction. Thus, the assertion is true.

5. **Understanding the Reason**: The reason states that "Friedel-Crafts reaction is an electrophilic substitution reaction."
- This statement is also true because Friedel-Crafts reactions are indeed a type of electrophilic substitution reaction.

6. **Evaluating the Relationship**: While both the assertion and reason are true, the reason does not explain why aniline does not undergo Friedel-Crafts reactions. The reason describes the nature of the Friedel-Crafts reaction but does not address the specific case of aniline.

7. **Final Answer**: Therefore, the correct option is B: "Assertion and reason both are true, but the reason is not the correct explanation for the assertion."