Strongest acid among the following is

Fluorine is a highly electronegative element, it withdraws electron from the neighbouring atoms. Therefore, more is the number of fluorine atoms attached, greater will be the acidity of the molecules. e.g. $\mathrm{FCH}_2 \mathrm{COOH}$ is more acidic than $\mathrm{CH}_3 \mathrm{COOH}$. Therefore, correct order of acidity is : $\mathrm{CF}_{3} \mathrm{COOH}$> \mathrm{CF} }_{2} \mathrm{HCOOH}>\mathrm{FCH}_{2} \mathrm{COOH} > \mathrm{CH} }_{3} \mathrm{COOH}