Which of the following is a strong base in an aqueous solution?

 ${\left({\mathrm{CH}}_3\right)}_2-\mathrm{NH}$ is a strong base in an aqueous solution among the given options. Because of the $+\mathrm{I}$ (inductive) action, aliphatic amines are stronger bases than ammonia. 

The electron-releasing alkyl groups enhance the electron density around the nitrogen, increasing the availability of the Ione pair of electrons to proton or Lewis acids and making the amine more basic. 

However, in tertiary amines, the crowding of alkyl groups covers the nitrogen atom from both sides, rendering it incapable of protonation. Tertiary amines are less basic than primary amines due to steric factors.

Hence, the correct answer is option D) ${\left({\mathrm{CH}}_3\right)}_2-\mathrm{NH}$.