Solution:

Option (iii) is the answer.

Reason:

Due to the electron releasing nature of the alkyl group, it $(R)$ pushes electrons towards nitrogen and thus makes the uncharged electrons pair available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilized due to the dispersal of the positive charge by the $+I$ effect of the alkyl group. Hence, alkylamines are stronger bases than ammonia.

Thus, the basic nature of aliphatic amines should increase with an increase in the number of alkyl groups.

thus, (iii) is the correct answer.