Arrange in decreasing order of acidity: (P) $\mathrm{CH}{}_3\mathrm{COOH}$, (Q) $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{OH}$, (R) $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{OH}$, (S) $\mathrm{CH}{}_3\mathrm{CH}{}_3$.

Acidity follows the stability of the conjugate base. The acetate ion has the negative charge delocalised over two oxygens, the phenoxide over oxygen and the ring, the ethoxide ion is localised on one oxygen, and the ethyl anion is on carbon. This gives P (pKa 4.8) > Q (10) > R (16) > S (about 50).