The correct option is A Kc=[H2] [I2][HI]2
The equilibrium constant of the reaction, Kc, is the ratio of the equilibrium concentrations of products over the equilibrium concentrations of reactants each raised to the power of their stoichiometric coefficients.
Thus, for reaction
2HI(g)⇌H2(g)+I2(g)
Kc=[H2] [I2][HI]2