The compound with least p^k_b value among the following is 

Basic strength in alkanamines is influenced by

A) +I effect of alkyl group

B) Steric hindrance

C) H-bonding

Based on +I effect 

Basic strength : ${\mathrm{R}}_3\mathrm{N}>{\mathrm{R}}_2\mathrm{NH}>\mathrm{R}-{\mathrm{NH}}_2>{\mathrm{NH}}_3$

Based on H-bonding and steric hindrance

Basic strength : ${\mathrm{R}}_3\mathrm{N}<{\mathrm{R}}_2\mathrm{NH}<\mathrm{R}-{\mathrm{NH}}_2<{\mathrm{NH}}_3$

Experimental ‘K_b’ values (Cumulative effect of all the above)

I) ${\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\mathrm{NH}>{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_3\mathrm{N}>{\mathrm{C}}_2{\mathrm{H}}_5-{\mathrm{NH}}_2>{\mathrm{NH}}_3$

II) ${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}>{\mathrm{CH}}_3{\mathrm{NH}}_2>{\left({\mathrm{CH}}_3\right)}_3\mathrm{N}>{\mathrm{NH}}_3$

In methyl amines there is no steric hindrance to H-bonding.