Equilibrium Constant Expression Steps:

• Write K as concentration of products raised to their coefficients divided by reactants raised to theirs.
• Products: [Fe(H2O)5OH]2+ has coefficient 2, H+ has 2, so numerator is ([Fe(H2O)5OH]2+)^2 [H+]^2.
• Reactants: [Fe(H2O)6]2+ has coefficient 1, Fe3+ has 2, so denominator is [Fe(H2O)6]2+ [Fe3+]^2.
• Combine to get full expression: ([Fe(H2O)5OH]2+)^2 [H+]^2 / [Fe(H2O)6]2+ [Fe3+]^2.

Why C is correct:

• Matches the definition of K_c for aqueous equilibria, where K_c = ∏ [products]^stoich / ∏ [reactants]^stoich.

Why the others are wrong:

• A: Omits coefficients for Fe3+ and [Fe(H2O)5OH]2+, ignores H+.
• B: Lacks ^2 on [Fe(H2O)5OH]2+ and omits [H+]^2 entirely.
• D: Misses ^2 on [Fe(H2O)5OH]2+ in numerator.

Final answer: C