1. Understand the question

We need to identify the reaction in which polysubstitution is a major drawback.

Polysubstitution means that after one substituent enters the aromatic ring, the product becomes even more reactive and undergoes further substitution, giving di-/tri-substituted products.

2. Check each option

A. Reimer–Tiemann reaction

This reaction is used for formylation of phenols, typically giving salicylaldehyde.

• It is not especially known for polysubstitution as the major drawback.
• Main issue is usually mixture of ortho/para products or moderate yield.

So, A is not correct.

B. Friedel–Crafts alkylation

In Friedel–Crafts alkylation, an alkyl group is introduced into an aromatic ring using an alkyl halide and a Lewis acid such as $\text{AlCl}_3$.

Example:

Now, an alkyl group is electron-donating due to the $+I$ effect and hyperconjugation. Therefore, once the first alkyl group enters the ring, the ring becomes more reactive than benzene itself.

So the product alkylbenzene undergoes further alkylation easily:

Hence, polysubstitution is a major drawback of Friedel–Crafts alkylation.

So, B is correct.

C. Acetylation of aniline

Acetylation of aniline forms acetanilide:

This is protection of the amino group, not an aromatic substitution reaction where polysubstitution is the main issue.

So, C is not correct.

D. Friedel–Crafts acylation

In Friedel–Crafts acylation, an acyl group is introduced:

The acyl group is electron-withdrawing due to the $-I$ and $-M$ effects, so after one acyl group enters, the ring becomes less reactive.

Therefore, polysubstitution does not occur easily in Friedel–Crafts acylation.

So, D is not correct.

3. Conclusion

The reaction in which polysubstitution is a major drawback is:

4. Comparison with stored correct answer

Stored correct answer: B

Our derived answer: B

They match.