Two substances are in equilibrium in a reversible chemical reaction. If the concentration of each substance is doubled, then the value of the equilibrium constant will be

Consider an elementary reaction $$A \Leftrightarrow B$$   and let equilibrium concentrations of $$A$$ and $$B$$ be $${{C_a}}$$ and $${{C_b}},$$
So equilibrium constant $$\left( K \right)$$ is $${K_1} = \frac{{{C_b}}}{{{C_a}}}$$
When the concentrations of both the species doubled equilibrium constant becomes $$K = \frac{{2{C_b}}}{{2{C_a}}} \Rightarrow K = {K_1}.$$     Hence remains constant.