If arg $\left(\frac{z-1}{z+1}\right)=\frac{\pi}{2}$, where $z$ is a complex number, locus of $z$ is
(a) $|\mathrm{z}|=1, \operatorname{Im}(\mathrm{z})>0$
(b) $|\mathrm{z}|=1, \operatorname{Im}(\mathrm{z})<0$
(c) $|z|=1$
(d) $|z-1|=1, \operatorname{Im}(z)>0$