Acid strength of carboxylic acids increases when electron-withdrawing groups (–I inductive effect) are present on the α-carbon, as they stabilize the carboxylate anion by dispersing negative charge through the chain. Halogens attached to the α-carbon exert a –I effect. The strength of the –I effect follows electronegativity: F > Cl > Br. Therefore, fluorine withdraws the most electron density from the carboxylate, providing the greatest stabilization of the conjugate base. FCH₂COOH (fluoroacetic acid, Ka ≈ 2.6 × 10⁻³) is the strongest acid among the options. Plain CH₃COOH (Ka = 1.8 × 10⁻⁵) is the weakest.