(c)
$\ \ A(g)\ \ \ \ \ \ \ \ + 3B(g) \rightleftharpoons 4C(g)$
1 1 0 Initial concentration
$(1 - x)$ $(1 - 3x)$ $4x$ Final concentration
(at equilibrium)
According to question, $1 - x = 4x$
$\therefore$ $x = \frac{1}{5}$
For above reaction,
$K_{c} = \frac{\lbrack C\rbrack^{4}}{\lbrack A\rbrack\lbrack B\rbrack^{3}} = \frac{(4x)^{4}}{(1 - x)(1 - 3x)^{3}}$
$K_{c} = \frac{\left( 4 \times \frac{1}{5} \right)^{4}}{\left( 1 - \frac{1}{5} \right)\left( 1 - 3 \times \frac{1}{5} \right)^{3}} = 8.0$