Addition of a catalyst in a system at equilibrium:

Among the following,the correct statement about the chemical equilibrium is

For a reversible reaction involving two reactants,if the concentration of the reactants is doubled,the equilibrium constant will:

When pure ammonia is placed in a vessel at constant temperature,at equilibrium:

$A$ liquid is in equilibrium with its vapour in a sealed container at a fixed temperature. The volume of the container is suddenly increased.
$(a)$ What is the initial effect of the change on vapour pressure?
$(b)$ How do rates of evaporation and condensation change initially?
$(c)$ What happens when equilibrium is restored finally and what will be the final vapour pressure?

Which of the following statements is not correct about the equilibrium constant $K_c$?