Number of structural isomers of $C_4H_{11}N$.

In a reaction of aniline,a coloured product $C$ was obtained as shown below:
$A$ $\xrightarrow{NaNO_2/HCl} B$ $\xrightarrow{N,N-dimethylaniline, \text{cold}} C$
The structure of $C$ would be:

$NBS \xrightarrow{KOBr} (A)$. The product $(A)$ is:

Amine compounds exhibit ............ isomerism.

Primary and secondary amines are distinguished by
$(i)$ Hinsberg test
$(ii)$ Carbyl amine test
$(iii)$ Hofmann mustard oil test
$(iv)$ $HNO_2$ test

The product of this Hoffmann bromamide reaction is: