Let $z=a-\large\frac{i}{2};$$ \alpha \; \in IR.$ Then $|i+z|^2-|i-z|^2=$

Question

\[\begin {array} {1 1} (a)\;2 & \quad (b)\;-2 \\ (c)\;4 & \quad (d)\;-4 \end {array}\]