The correct option is A (CH3)3CNH2
In a one degree amine, only one hydrogen atom of NH3 is replaced by alkyl group.
So option (a) is correct because one of the hydrogens has been replaced by (CH3)3C− group.
General formula for primary amine is RNH2
The kind of alkyl group does not matter in deciding the type of amine, what matters is the number of hydrogen that are replaced.