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

The acidic strength of acetic acid is lower than that of formic acid, as the methyl group is an electron releasing group and destabilizes the carboxylate anion. Chlorine and fluorine are electron withdrawing groups. They stabilize the carboxylate anion. Hence, trichloroacetic acid and trifluoro acetic acid are more acidic than formic acid. Fluorine is more electron withdrawing than chlorine. Hence, trichloroacetic acid (A) is less acidic than trifluoroacetic acid (B).
Therefore, the correct order of decreasing acid strength is B > A > D > C.