For the reaction 
$C_{\left(g\right)}+D_{\left(g\right)}\rightleftharpoons A_{\left(g\right)}+B_{\left(g\right)}$ , $k_c=4$ when one mole of each A, B, C and D are taken in a one litre vessel. Equilibrium concentration of C would be

$\begin{array}{l}C+ D\rightleftharpoons A+ B\\ 11 11\\ 1-x1-x1+x1+x\end{array}$

$k_c=\frac{\left[A\right]\left[B\right]}{\left[C\right]\left[D\right]}\Rightarrow 4=\frac{{\left(1+x\right)}^2}{{\left(1-x\right)}^2}$

$\frac{1+x}{1-x}=2\Rightarrow 2-2x=1-x$

$\Rightarrow 3x=1\Rightarrow x=\frac{1}{3}$

$\left[A\right]=1+\frac{1}{3}=\frac{4}{3} ; \left[B\right]=\frac{4}{3}$

$\left[C\right]=1-\frac{1}{3}=\frac{2}{3} ; \left[D\right]=\frac{2}{3}$