The increasing order of acidic strength among the following compounds is I . Benzoic acid II . 4 -…

Since electron-donating groups decrease the acid strength, $4-$methoxy benzoic acid is a weaker acid than benzoic acid. Further, since electron-withdrawing groups increase the acid strength, both $4-$nitrobenzoic acid and $3, 4-$dinitrobenzoic acid are stronger acids than benzoic acid.

Further, due to the presence of an additional ${\mathrm{NO}}_2$​ at $\mathrm{m}-$position with respect to $\mathrm{COOH}$ group, $3, 4-$dinitrobenzoic acid, the overall acid strength increase in the order:

$4-$methoxy benzoic acid $<$ Benzoic acid $<$ $4-$nitrobenzoic acid $<$ $3, 4-$dinitrobenzoic acid