Dielectrics question from JEE Main 2019 (9 Jan, Shift 2)

Answer: (a)

1. Each half (area $A/2$) has two layers of thickness $d/2$ in series: each layer gives $\frac{K\epsilon_0 (A/2)}{d/2} = K\frac{\epsilon_0 A}{d}$.
2. Top: $\frac{K_1 K_2}{K_1 + K_2}\frac{\epsilon_0 A}{d}$; bottom: $\frac{K_3 K_4}{K_3 + K_4}\frac{\epsilon_0 A}{d}$.
3. The two halves are in parallel: $K = \frac{K_1 K_2}{K_1 + K_2} + \frac{K_3 K_4}{K_3 + K_4}$
4. So the answer is (a).