Preferred Site for Electrophilic Attack on Given Aromatic Molecule

To determine the preferred site for electrophilic aromatic substitution in the molecule $x$ (benzanilide, $\text{Ph-CO-NH-Ph}$), we analyze the electronic effects of the substituents on both aromatic rings:

1. Comparison of Ring Activation:

   • Benzoyl Ring (containing sites $r$ and $p$): This ring is directly attached to the carbonyl group ($-\text{C(=O)}-$). The carbonyl group acts as an electron-withdrawing group via resonance ($-M$) and inductive effects ($-I$). As a result, this ring is deactivated toward electrophilic aromatic substitution.
   • Anilidic Ring (containing sites $s$ and $u$): This ring is directly attached to the nitrogen atom ($-\text{NH}-$). The lone pair of electrons on the nitrogen atom can undergo resonance with the $\pi$-system of the benzene ring ($+M$ effect), significantly increasing the electron density. Thus, this ring is strongly activated toward electrophilic aromatic substitution relative to the benzoyl ring.

2. Regioselectivity on the Activated Ring:

   • The $-\text{NH-CO-Ph}$ group is an ortho/para-directing group.
   • Position $s$ corresponds to the ortho position relative to the $-\text{NH-CO-Ph}$ group.
   • Position $u$ corresponds to the para position relative to the $-\text{NH-CO-Ph}$ group.
   • Due to the bulky nature of the $-\text{NH-CO-Ph}$ group, the ortho position ($s$) experiences significant steric hindrance.
   • Therefore, electrophilic attack occurs with high selectivity at the sterically unhindered para position ($u$).

Thus, the preferred site for the attack of the electrophile is predominantly at "$u$".

Correct Option: D (Predominantly at "$u$")