Answer:
I think it's b if not then sorry
H2O is the missing reactant.
Just a caveat: this equation isn’t balanced.
Answer: ![K_c=\frac{[CH_3Cl]\times [OH^-]}{[CH_3OH]\times [Cl^-]}](https://tex.z-dn.net/?f=K_c%3D%5Cfrac%7B%5BCH_3Cl%5D%5Ctimes%20%5BOH%5E-%5D%7D%7B%5BCH_3OH%5D%5Ctimes%20%5BCl%5E-%5D%7D)
Explanation:
Equilibrium constant is the ratio of the concentration of products to the concentration of reactants each term raised to its stochiometric coefficients. Pure solids are assumed to have a concentration of 1.
The given balanced equilibrium reaction is:

The expression for equilibrium constant for this reaction will be,
![K_c=\frac{[CH_3Cl]\times [OH^-]}{[CH_3OH]\times [Cl^-]}](https://tex.z-dn.net/?f=K_c%3D%5Cfrac%7B%5BCH_3Cl%5D%5Ctimes%20%5BOH%5E-%5D%7D%7B%5BCH_3OH%5D%5Ctimes%20%5BCl%5E-%5D%7D)
Thus the equilibrium constant expression for this reaction is ![K_c=\frac{[CH_3Cl]\times [OH^-]}{[CH_3OH]\times [Cl^-]}](https://tex.z-dn.net/?f=K_c%3D%5Cfrac%7B%5BCH_3Cl%5D%5Ctimes%20%5BOH%5E-%5D%7D%7B%5BCH_3OH%5D%5Ctimes%20%5BCl%5E-%5D%7D)
The answer is Thermal Energy :)