(s) about substituted benzenes.

A is incorrect: an electron-donating methyl group destabilises the phenoxide ion, so $p$-cresol ($pK_a\approx10.3$) is weaker than phenol ($pK_a\approx10.0$). B is correct: in mildly alkaline medium the phenoxide ion, a strongly activated nucleophile, attacks the diazonium ion at the para position (azo coupling). In strongly acidic medium phenol is not ionised and coupling fails, and in strongly basic medium the diazonium ion converts to diazotate. C is incorrect: in the strongly acidic nitrating mixture aniline is protonated to the anilinium ion, a meta director, so a large fraction of the $m$-isomer forms along with oxidation products; hence the product is not exclusively para. D is correct: Reimer–Tiemann reaction: $\mathrm{CHCl_3+OH^-\to{:}CCl_2}$, attack by the phenoxide ortho carbon, then hydrolysis to the o-hydroxybenzaldehyde. Hence B and D.