Le Chatelier's principle on heterogeneous equilibrium

Steps:

• Identify the equilibrium: 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g); Kp = (P_{H₂})⁴ / (P_{H₂O})⁴.
• Mass of Fe changes if equilibrium shifts, consuming or producing solid Fe.
• Evaluate A: Adding H₂O at constant P increases P_{H₂O}, shifting right to consume Fe.
• Note D is unspecified, making full analysis impossible.

Why D is correct:

• Not enough information (choice D blank, cannot verify effect).

Why the others are wrong:

• A: Shifts equilibrium right, decreasing Fe mass.
• B: Δn_g = 0, so no shift; Fe mass unchanged.
• C: Catalyst equalizes rates without shifting position; Fe mass unchanged.

Final answer: Not enough information.