Answer: Normal, gravity, friction, and air resistance
Step-by-step explanation:
Gravity always acts on an object. It is responsible for pulling the sledder towards the center of the earth.
Normal force is what keeps the sledder from sinking into the snow and prevents gravity from doing that.
Friction is what allows the sledder to slow down. It acts in the opposite direction of the sledder's motion.
Air resistance (or drag) is also what allows the sledder to slow down. It acts in the opposite of the sledder's motion. Air resistance is going to have more of an impact on the sledder as the velocity increases (
= -kv).