Which of the following is the stronger acid?

Acetic acid  Propanoic acid Isobutyric acid 2,2-Dimethylpropanoic acid

Acetic acid

As the size of alkyl group (electron releasing) increases, the electron density on the O of OH group increases, and makes release of H+ ion more difficult. Thus, the release of proton from acetic acid will be easier compared to the higher acids, and is therefore the stronger acid.