Arrange in decreasing order of acid strength: (P) $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{COOH}$, (Q) $\mathrm{CH}{}_2=\mathrm{CH}-\mathrm{COOH}$, (R) $\mathrm{HC}\equiv \mathrm{C}-\mathrm{COOH}$.

The carbon attached to $\mathrm{COOH}$ is sp, sp2 or sp3, and its electronegativity increases with s character. A more electronegative carbon withdraws electrons (-I) from the carboxylate more strongly. This gives propiolic (pKa 1.9) > acrylic (4.3) > propanoic acid (4.9).