Polysubstitution is a major drawback in:

We begin by recalling the meaning of the term $$\text{polysubstitution}$$. In aromatic electrophilic substitution, polysubstitution means that after the first substituent is introduced, further substitutions keep taking place on the same benzene ring, often yielding di- or tri-alkylated products rather than the desired mono-alkylated compound. This situation is undesirable because it lowers the yield of the required mono-substituted product and complicates purification.

Now we examine each of the given reactions one by one and analyse whether polysubstitution is an inherent drawback.

First, consider the Reimer-Tiemann reaction. In this reaction, phenol is treated with chloroform and a strong base to give salicylaldehyde. The reaction proceeds through a dichlorocarbene intermediate that inserts at the ortho and para positions. Once the formyl group $$\left( -\!\!CHO \right)$$ is installed, the ring becomes deactivated because the $$-CHO$$ group is $$-M$$ and $$-I$$ withdrawing, so further electrophilic attack is suppressed. Therefore polysubstitution is not a major issue in the Reimer-Tiemann reaction.

Next, we analyse Friedel-Crafts alkylation, in which an alkyl halide $$R-X$$ reacts with benzene in the presence of a Lewis acid such as $$AlCl_3$$ to give an alkylbenzene. The key mechanistic step is the generation of a carbocation $$R^+$$ (or a Lewis-acid-complexed equivalent), which is a very strong electrophile. After the first alkyl group attaches to the benzene ring, it exerts a $$+I$$ inductive effect and a $$+H$$ hyperconjugative effect, both of which activate the ring and make it still more electron-rich. Because of this increased activation, the freshly formed alkylbenzene reacts faster than the original benzene, so more alkyl cations attack it in subsequent steps. This chain of events produces di- and tri-alkylated products very easily. Thus polysubstitution is indeed a major drawback in Friedel-Crafts alkylation.

Now we look at the acetylation of aniline, typically performed with acetic anhydride $$\left( (CH_3CO)_2O \right)$$. In this reaction the amino group $$-NH_2$$ is converted into an amide $$-NHCOCH_3$$. The product contains the $$-NHCOCH_3$$ group, which is less activating than the parent $$-NH_2$$ group because the lone pair on nitrogen becomes partially delocalised into the carbonyl. Therefore the ring is not strongly activated toward further electrophilic attack, and polysubstitution is not a significant problem.

Finally, Friedel-Crafts acylation introduces an acyl group $$RCO-$$ using an acyl chloride $$RCOCl$$ and a Lewis acid such as $$AlCl_3$$. The acylium ion $$RCO^+$$ is generated as the electrophile. After the first acyl group enters the ring, the newly formed ketone group $$-COR$$ is strongly $$-M$$ and $$-I$$ withdrawing, which deactivates the ring. That deactivation greatly diminishes the likelihood of further substitutions, so polysubstitution is almost never observed in Friedel-Crafts acylation.

Comparing all four possibilities, we see that only Friedel-Crafts alkylation suffers from the intrinsic problem that the first substitution step makes the aromatic ring more reactive instead of less. This leads to the formation of multiple alkylated products unless stringent reaction control (large excess of benzene, low temperature, etc.) is employed.

Hence, the correct answer is Option 2.