Acid strength increases with electron-withdrawing inductive effect of substituents closer to the –COOH group. In option B, Cl is at the $\alpha$-carbon (adjacent to –COOH), providing maximum inductive withdrawal. In option C, Cl is at the $\gamma$-position (far from –COOH), with much weaker inductive effect. Formic acid (A) is stronger than acetic acid (D) but weaker than $\alpha$-chloro acid.