The answer will be the molecule that is the most basic, or the one that has the greatest ability to donate electron density.
a) CH3CONH2 --> The CO represents a carbonyl group and the adjacent N exhibits resonance with the C=O double bond, delocalizing the electrons and making this molecule's ability to donate electron density worse. Probably not your answer.
B) CH3CH2NH2 --> This answer is okay. The N has two lone pairs and there is no electron delocalization. Maybe
C) (CH3CH2)2NH2 --> This answer is better than the last group because it is more substituted. As you know, alkyl groups donate electron density so because this one has more alkyl groups, the N in this case will have more electron density than in B. Better answer.
D) C5H6NH2 --> The N in this case exhibits resonance with the benzene ring it is attached to. For the same reasoning in A, this is probably not your best answer.
Therefore, because the N in C can donate the most electron density, it is your best answer.
Hope this helps.
