Answer
a) Amines are the functional group having N atom & the remaining valency are filled with either 2 H atoms ($-NH_{2}$) or one alkyl group and one H atom ($-NHR$) or two alkyl groups ($-NR_{2}$).
b) Ammonia($NH_{3}$) has the following structure as figure below. In this structure we see that N atom bonded with 3 H atoms by three single bonds. Now, if we substitute an alkyl group in place of one of H atoms it becomes $R-NH_{2}$ which is an example of primary amine where the amine group is $-NH_{2}$. So, we can tell that amines are derived from ammonia i.e. amines described as derivatives of ammonia.
Work Step by Step
R = Alkyl groups ($-CH_{3}, -CH_{2}CH_{3} $ etc.)
Explanation of the said question has been discussed in the answer section above.
