ANS:A - an oily surface.

Dropwise condensation of steam on a cooling surface is promoted by an oily surface. When the surface is coated with a thin layer of oil or a similar substance, it creates a non-wetting surface for the condensing steam droplets. This non-wetting surface encourages the formation of discrete droplets, known as dropwise condensation, instead of a continuous film of condensate, known as filmwise condensation. Dropwise condensation is preferred over filmwise condensation because it typically leads to higher overall heat transfer coefficients and reduced fouling on the surface. Therefore, promoting dropwise condensation by using an oily surface is a common strategy in various heat transfer applications to enhance efficiency and performance.