Dielectrics question from MHT CET 2026 (19 May, Shift 1)

Answer: (d)

1. Initially two equal capacitors in series: $C_i = \frac{C}{2}$.
2. After filling one: $C_f = \frac{KC \cdot C}{KC + C} = \frac{KC}{K+1}$.
3. $\Delta C = \frac{KC}{K+1} - \frac{C}{2} = \frac{C(2K - K - 1)}{2(K+1)} = \frac{C}{2}\left[\frac{K-1}{K+1}\right]$
4. So the answer is (d).