Concept:Both solid friction and viscosity are frictional effects that oppose relative motion.Explanation:Solid friction acts between two solid surfaces in contact and always opposes motion or attempted motion.Viscosity acts inside a fluid and opposes relative motion between its layers.While solid friction depends on normal reaction, viscosity does not.However, the common feature is that both act in a direction opposite to motion.This means they both resist the movement happening between surfaces or layers.Answer:D. in opposition to motion.