(b) Suppose the charge on the capacitor at time $t$ is $Q$, the electric field between the plates of the capacitor is $E = \frac{Q}{\varepsilon_{0}A}$. The flux through the area considered is
$\phi_{E} = \frac{Q}{\varepsilon_{0}A}.\frac{A}{2} = \frac{Q}{2\varepsilon_{0}}$
$\therefore$ The displacement current
$i_{d} = \varepsilon_{0}\frac{d\phi_{E}}{dt} = \varepsilon_{0}\left( \frac{1}{2\varepsilon_{0}} \right)\frac{dQ}{dt} = \frac{i}{2}$