Answer
$\frac{[COCl_{2}]}{[Cl_{2}][CO]}$
Work Step by Step
The equilibrium constant is the ratio of the concentration of products to the concentration of reactants, each raised to power their stoichiometric coefficients.
In this case, the reactants are written in the numerator and the product is written in the denominator, which is incorrect.
The reactants are written in the denominator and the products in the numerator.
Therefore the equilibrium constant becomes, $\frac{[COCl_{2}]}{[Cl_{2}][CO]}$.