(d)
$A\ \ \ \ \ \ + \ \ \ \ \ \ \ \ B\ \ \ \ \ \ \ \rightleftharpoons \ \ \ \ \ \ C\ \ \ \ + \ \ \ \ \ D$
1 1 0 0 Initially
$(1 - 3x)$ $(1 - 3x)$ $3x$ $3x$ At equilibrium (given)
At equilibrium, the remaining moles of $A$ is $x$, because $3x$ moles of $C$ are produced.
$\Rightarrow$ $1 - 3x = x$
$\therefore$ $x = \frac{1}{4}$
Equilibrium constant,
$K_{c} = \frac{\lbrack C\rbrack\lbrack D\rbrack}{\lbrack A\rbrack\lbrack B\rbrack} = \frac{3x.3x}{(1 - 3x)^{2}}$
On putting the value of $x$, we get,
$K_{c} = \frac{9 \times \frac{1}{16}}{1 + \frac{9}{16} - \frac{6}{4}} = \frac{9}{1} = 9$