Find the strongest acid among the following compounds is:

${\mathrm{H}}_3{\mathrm{N}}^{\oplus }-{\left({\mathrm{CH}}_3\right)}_2-\mathrm{COOH}$  has strongest acid due to strong -I Effect $-\stackrel{\oplus }{\mathrm{N}}{\mathrm{H}}_3$.

As  -I effect increases , dissociation of COOH (COO^-  & H⁺ ) increases , acidic nature increases.

Order of -I effect is    $-{\mathrm{H}}_3{\mathrm{N}}^{\oplus }>-\mathrm{F}>-\mathrm{COOH}>-{\mathrm{CH}}_3$