Answer:
No one is correct. The correct expression is:
Keq = [H₂]²  . [O₂]² / [H₂O]²
Explanation:
To build the Keq expression in a chemical equilibrium you must consider the molar concentrations of reactants / products, and they must be elevated to the stoichiometric coefficient.
The balance reaction is:
2 H₂O (g)  ⇄  2 H₂ (g)  +  O₂ (g)
Keq = [H₂]²  . [O₂]  / [H₂O]²
In opposite side: 2 H₂ (g)  +  O₂ (g)  ⇄  2 H₂O (g)
Keq =  [Hâ‚‚O]² / [Hâ‚‚]²  . [Oâ‚‚] Â