The decreasing order of electrical conductivity of the following aqueous solutions is : 0.1 M Formic acid (A); 0.1 M Acetic acid (B) and 0.1 M Benzoic acid (C)

Acidic strength of a solution is defined as the concentration of the H ions produced when an acid and a base are brought together in water. 

Inversely, the acidic strength can be defined as the amount of an acid needed to react with sufficient base to form one equivalent of H ions.

0.1 M Formic acid (A)

 0.1 M Benzoic acid (C) 

0.1 M Acetic acid (B)

Hence, option C is correct.