ANS:B - oily

I see where the confusion might be. Let me correct that. Dropwise condensation is actually promoted on a smooth surface, not an oily one. Here's why:

1. Surface Smoothness: A smooth surface provides fewer nucleation sites for condensate to form compared to a rough surface. With fewer sites for droplets to form, condensate is more likely to collect into larger droplets and shed off the surface as gravity overcomes surface tension. This shedding action enhances heat transfer efficiency by constantly exposing fresh surface area for condensation to occur.
2. Reduced Adhesion: On a smooth surface, the adhesive forces between the condensed droplets and the surface are minimized. This allows the droplets to easily detach from the surface, promoting dropwise condensation.

While an oily surface may indeed reduce adhesion of the condensate to some extent due to the presence of a thin layer of oil, it may not provide the ideal conditions for promoting dropwise condensation. The presence of oil can also introduce other complications such as contamination of the condensate. Therefore, a smooth surface is generally considered most conducive to promoting dropwise condensation.