(d) $C_{air} = \frac{\varepsilon_{0}A}{d},$ with dielectric slab $C^{'} = \frac{\varepsilon_{0}A}{\left( d - t + \frac{t}{K} \right)}$
Given $C^{'} = \frac{4}{3}C \Rightarrow \frac{\varepsilon_{0}A}{\left( d - t + \frac{t}{K} \right)} = \frac{4}{3} \times \frac{\varepsilon_{0}A}{d}$
$\Rightarrow K = \frac{4t}{4t - d} = \frac{4(d/2)}{4\lbrack(d/2) - d\rbrack} = 2$