(c)
$A\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ + \ \ \ \ B\ \ \ \ \ \ \rightleftharpoons \ \ \ C\ \ \ + \ \ \ D$
Initial $x$ mol $x$ mol
At equilibrium $x - \frac{x}{3} = \frac{2x}{3}$ $\frac{2x}{3}$ $\frac{x}{3}$ $\frac{x}{3}$
Hence,
$K_{c} = \frac{\lbrack C\rbrack\lbrack D\rbrack}{\lbrack A\rbrack\lbrack B\rbrack} = \frac{\left( \frac{x}{3V} \right)\left( \frac{x}{3V} \right)}{\left( \frac{2x}{3V} \right)\left( \frac{2x}{3V} \right)} = \frac{1}{4} = 0.25$