Which one of the following group shows maximum -I effect

-I effect is shown by electron withdrawing groups.

Here, in - ${\mathrm{NMe}}_3$, due to steric hindrance the $\mathrm{N}-\mathrm{R}$ bond length increases and it becomes difficult for the alkyl group to stabilize the positive charge on the nitrogen atom.

So, the order of -I effect will be

${\mathrm{NMe}}_3>{\mathrm{NHMe}}_2>{\mathrm{NH}}_2\mathrm{Me}>{\mathrm{NH}}_3$

Thus, $-{\mathrm{NMe}}_3$ shows maximum -I effect.

Hence, the correct option is (D).