Topical corticosteroid potency is not just about the molecule; the base it is mixed into matters a great deal. The reason is penetration. Whatever helps the steroid cross the skin barrier raises its effective strength at the same concentration.
Picture how each preparation behaves on skin. An ointment is oily and forms a sealing layer over the surface. This occlusion holds water in the outer skin, softens and hydrates the stratum corneum, and lets far more drug diffuse through. That occlusive boost is why an ointment delivers the strongest effect of the four.
A cream is a water-and-oil emulsion that feels lighter and is only partly occlusive, so it pushes less drug through. Gels and lotions are even lighter, often evaporate or dry on the skin, and provide the least occlusion, so they give the weakest penetration of all.
Lining them up from strongest to weakest gives ointment, then cream, then the gel and lotion forms. The winner for potency is therefore the ointment.
\[\boxed{\text{Ointment (option C)}}\]