Which of the following factors affect friction?

Friction is affected by the nature of the surfaces in contact (roughness, material), which determines the coefficient of friction. The weight of the object influences the normal force, which directly affects the frictional force (friction is proportional to normal force). Although in the basic model of kinetic friction the force is independent of speed, in real-world scenarios and for static friction, motion or speed can have an effect (e.g., speed-dependent friction in fluids or slight variations in solid friction). Therefore, all three factors—nature of surface, weight (normal force), and motion/speed—can affect friction, making option D the most comprehensive answer.