Which is the decreasing order of acidity in,

I) HCOOH   II) CH₃COOH     III) CH₃CH₂COOH   IV) C₆H₅COOH

Among given based on I-effects & R-effect is less acidic than HCOOH & order of acidity is

stabilize the Benzene.Acidity increases if 
1. More minus I effect is present.
2. The resulting ion is stable.
3. The proton is donated fast.
4.Conjugate base must be stable.

The general order of acidity is,
RCOOH>Phenol>Water>Alcohols>yne>ene>ane.