Friction is classified according to the physical mechanism resisting relative motion between two bodies. When two surfaces are separated by a continuous lubricant layer, the classification is film friction (also called fluid friction), which is the correct answer.

In film (fluid) friction the mating surfaces are completely separated by a film of lubricant, so there is no direct metal-to-metal contact of the asperities. The resistance to motion no longer comes from surface roughness but instead from the viscosity (internal shearing) of the fluid — adjacent layers of the lubricant slide over one another and the shear stress developed is proportional to the velocity gradient and the fluid's viscosity. This is the ideal lubrication regime (hydrodynamic/thick-film lubrication) sought in journal bearings, because it gives very low friction and virtually no wear of the solid surfaces.

The other options describe different friction mechanisms and do not involve a separating fluid film:

• Dry friction (Coulomb friction) occurs between un-lubricated solids in direct contact, arising from interlocking and adhesion of surface asperities — the exact opposite of a lubricated condition.
• Rolling friction is the resistance encountered when one body rolls over another (as a wheel or ball bearing does); it is governed by elastic deformation at the contact, not by a lubricant film.
• Static friction is the friction that must be overcome to start motion between surfaces at rest; it describes a state of motion (impending motion), not the presence of a fluid.

Therefore, when the surfaces are lubricated with a fluid, the friction present is film friction.