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

The acidity of carboxylic acids depends on the stability of the carboxylate anion formed after losing a proton. This stability is influenced by Inductive Effects:

1. Electron-Withdrawing Groups (EWG): Groups like halogens exert a negative inductive effect (-I), dispersing the negative charge on the anion and increasing stability (and acidity). Fluorine is more electronegative than Chlorine, so the -I effect of $-CF_3$ is stronger than that of $-CCl_3$. Thus, Trifluoroacetic acid (B) > Trichloroacetic acid (A).
2. Electron-Donating Groups (EDG): Alkyl groups like methyl ($-CH_3$) exert a positive inductive effect (+I), intensifying the negative charge and destabilizing the anion (decreasing acidity). Formic acid ($HCOOH$) has no alkyl group, while Acetic acid ($CH_3COOH$) has a methyl group. Thus, Formic acid (D) > Acetic acid (C).

Combining these trends, the order is B > A > D > C.