The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base

Basic character of amines can be better understood in terms of their K_b and pK_b, Larger the value of K_b or smaller the value of pK_b , stronger is the base.

The order of basic strength in case of methyl substituted amines and  in aqueous solution depends on inductive effect ,steric effect and solvation effect.

Hence, the correct basicity order is

secondary amines >primary amines> tertiary amines

Hence, ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$ is less basic than ${\mathrm{CH}}_3{\mathrm{NH}}_2$.

Therefore, pK_b value is more for ${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$.

C and D are less basic than A and B due to the presence of phenyl group in C and D which causes -I effect.

But C is more basic than D due to lesser steric hindrance.Hence, the pK_b value is more D

So, the correct order of pK_b values is