H+donors are acids. Electron donating groups increase acidity and electron withdrawing groups decrease acidity.
In CH3COOH and CH3CH2COOH : Alkyl groups donate electrons towards O-H group due to +I effect and increases the bond strength of C−H bond. ∴ Acidity is decreased.
In C6H5COOH : Phenyl group attracts the electrons away from benzene ring and hence, O-H bond strength decreases and therefore acidity increases. HCOOH : It has neither +I or −I effect. So its acidity is not affected. ∴C6H5COOH is most acidic.