Ka for  formic acid and  acetic acid  are 2.1 × 10-4 and 1.1 × 10-5 respectively. The relative strength of acids is

Relative strengths of weak acids = Ka1Ka2Assume C1 and C2 are same (Although not given).∴  Relative strength = Ka1Ka2 = 2.1 × 10-41.1 × 10-5Relative strength for HCOOH to CH3COOH = 4.37 : 1.