a
The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows:

\(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{NH}>\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{N}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NH}_{2}>\mathrm{NH}_{3}\)

\(\left(\mathrm{CH}_{3}\right)_{2}\mathrm{NH}>\mathrm{CH}_{3} \mathrm{NH}_{2}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{N}>\mathrm{NH}_{3}\)