(b)
Let $\mathcal{z =}x + iy$
$\Rightarrow \ \mathcal{z}^{2} = x^{2} - y^{2} + 2i\ xy$
$\Rightarrow Re\left( \mathcal{z}^{2} \right) = Re(x^{2} - y^{2} + 2ixy)$
$\Rightarrow 1 = x^{2} - y^{2}\ \ \ \ \ \ \ \ \ \ \lbrack\because Re\left( \mathcal{z}^{2} \right) = 1$ (given)]