Which of the following have least ${\mathrm{P}}^{{\mathrm{k}}_{\mathrm{a}}}$ value?

Lower the value of ${\mathrm{P}}^{{\mathrm{k}}_{\mathrm{a}}}$, stronger will be the acid. The order of acidity of acids is ${\mathrm{CF}}_3\mathrm{COOH}>{\mathrm{CCl}}_3\mathrm{COOH}>{\mathrm{CCl}}_2\mathrm{HCOOH}>{\mathrm{CH}}_3\mathrm{COOH}$
The –I effect of −F is >−Cl 
As the number of –I groups increases, acidic strength increases.
${\mathrm{P}}^{{\mathrm{k}}_{\mathrm{a}}}$of CF₃COOH is 0.23