Polysubstitution is a major drawback in:

Step 1: Understand Polysubstitution

Polysubstitution = once the first group is introduced, the product is MORE reactive than the starting material, leading to multiple substitutions. Step 2: Evaluate Each Reaction

(a) Friedel-Crafts alkylation: When an alkyl group ($\ce{-R}$) is introduced, it is electron-donating (activating, ortho/para director). The monoalkylated product is MORE reactive than benzene, so further alkylation occurs easily, giving di-, tri-alkylated products. Polysubstitution is a major drawback.

(b) Reimer-Tiemann: Formylation of phenol; limited by the specific conditions and stoichiometry. Not primarily known for polysubstitution.

(c) Acetylation of aniline: The $\ce{-NHCOCH3}$ group is less activating than $\ce{-NH2}$. Moderate; monoacetylation is controlled. Not the primary example.

(d) Friedel-Crafts acylation: Acyl group ($\ce{-COR}$) is electron-withdrawing (deactivating). The monoacylated product is LESS reactive than benzene. Polysubstitution does NOT occur. This is why acylation is preferred over alkylation.

Answer: (a) Friedel-Crafts alkylation.

Key Points:

• FC alkylation: monoalkyl product more reactive = polysubstitution occurs
• FC acylation: monoacyl product deactivated = monosubstitution only
• This is one key advantage of acylation over alkylation followed by Clemmensen