Which carboxylic acid has the highest $K_a$ (i.e. is the most acidic)?

1. The acidity of a substituted carboxylic acid is governed primarily by the INDUCTIVE EFFECT of the substituent on the carboxylate stability. 2. Among the halogens, the ELECTRONEGATIVITY order is F > Cl > Br > I. Higher electronegativity → stronger electron-withdrawing inductive effect → more stable conjugate base → higher acidity. 3. So among the $\alpha$-halo propanoic acids, the FLUORO derivative is most acidic. 4. The numbers: $K_a$ values (approximate, NCERT Table 8.4 style): unsubstituted propanoic $\approx 1.3\times 10^{-5}$; $\alpha$-F $\approx 2\times 10^{-3}$; $\alpha$-Cl $\approx 1.4\times 10^{-3}$; $\alpha$-Br $\approx 1.3\times 10^{-3}$. F-substitute wins. 5. Option A has no substituent at all and is least acidic. Options C and D weaker than B for the reason in step 2. _Source: NCERT Class 12 Chemistry Part 2, Ch 8, §8.6 (Carboxylic acids — effect of substituents on $K_a$) + Table 8.4, p. 18–19._