The strongest acid amongst the following compounds is:

Acidic strength increases with the presence of electron-withdrawing groups ($-I$ effect) and decreases with electron-donating groups ($+I$ effect).Chlorine has a $-I$ effect. The closer the $-I$ group to the $-\text{COOH}$ group, the stronger the acid.In ${\text{CH}}_3{\text{CH}}_2\text{CH}(\text{Cl})\text{COOH}$, Cl is at the $\alpha$ -carbon.In ${\text{ClCH}}_2{\text{CH}}_2{\text{CH}}_2\text{COOH}$, Cl is far away at the $\gamma$ -carbon.$\alpha$ -substituted chloro-acids are much stronger than formic acid or acetic acid.