This question tests AP Physics C: Mechanics skills, specifically understanding of kinetic and static friction and their roles in force dynamics. Kinetic friction acts on moving objects with a constant magnitude equal to μk times the normal force, independent of the object's speed or applied force. In this scenario, a 10.0 kg sled is already moving on packed snow with μk = 0.10, and a student pulls with 30 N horizontally. Choice A (9.8 N) is correct because kinetic friction equals μk × N = μk × mg = 0.10 × 10.0 × 9.8 = 9.8 N, regardless of the 30 N applied force. Choice B (19.6 N) incorrectly uses the static coefficient or doubles the kinetic friction, while choice C uses the applied force magnitude. Students should understand that kinetic friction depends only on the coefficient and normal force, not on the applied force or velocity. Draw free-body diagrams showing all forces and emphasize that kinetic friction opposes motion with constant magnitude.