The starting material p-nitrotoluene is first reduced by P (either H${}_2$/Pd in ethanol or Sn/HCl, both reduce –NO${}_2$ to –NH${}_2$) to give Q = p-toluidine (4-methylaniline). So options (A) and (B) for P are both viable, but (D) misidentifies Q as 4-nitrobenzoic acid.

Diazotisation by R (NaNO${}_2$/HCl or HNO${}_2$) at low temperature gives S = p-tolyl diazonium chloride. Both (A) and (B) describe valid choices for R.

Hydrolysis of S with warm H${}_2$O gives T = p-cresol (4-methylphenol).

Conversion of S to benzoic acid by U requires two operations: replacement of the diazonium group by H (using H${}_3$PO${}_2$ or CH${}_3$CH${}_2$OH), giving toluene, and then oxidation of the methyl group with alkaline KMnO${}_4$ followed by acidification, giving benzoic acid. Both forms of U in (A) and (C) are correct.

Therefore options (A), (B) and (C) all describe valid steps. Option (D) is wrong because Q in this sequence is the amine, not the carboxylic acid.