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

Answer: Option A

Consider an elementary reaction $$A\iff B$$   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 $$K=\frac{2C_b}{2C_a}\Rightarrow K=K_1.$$     Hence remains constant.