Dielectrics question from JEE Main 2017 (9 Apr, Shift 1)

Answer: (c)

1. The wires join D to E and C to the right node, so all three capacitors sit between the same two nodes (in parallel) at the same potential difference.
2. Same $V$ needs each capacitance unchanged: a slab of thickness $t$ acts like air of thickness $t/K$, so the gap must grow by $t\left(1 - \frac{1}{K}\right)$.
3. $3\left(1 - \frac{1}{K}\right) = 2.4 \Rightarrow \frac{1}{K} = 0.2 \Rightarrow K = 5$
4. So the answer is (c) 5.