The correct order of decreasing acid strength of trichloroacetic acid (A), trifluoroacetic acid (B), acetic acid (C) and formic acid (D) is:

Acid strength is increased by electron-withdrawing groups via the inductive effect ($-I$ effect).${\text{CF}}_3\text{COOH}$ (B): Fluorine is the most electronegative element, providing the strongest $-I$ effect. It is the strongest acid.${\text{CCl}}_3\text{COOH}$ (A): Chlorine is electronegative but less so than Fluorine. It is very strong but less than B.$\text{HCOOH}$ (D): Formic acid is stronger than acetic acid because the methyl group in acetic acid is electron-donating ($+I$ effect), which destabilizes the carboxylate ion.${\text{CH}}_3\text{COOH}$ (C): Weakest due to the $+I$ effect of $-{\text{CH}}_3$.Order: $B>A>D>C$.