Answer: the force of static friction of the sleigh on the crate
Step-by-step explanation:
The force that is responsible for the crates increase in speed up the hill is the force of static friction of the sleigh on the crate.
When a light horizontal push is applied and which isn't enough to move the box, the static friction force will also be directly opposite to the push.
When an individual pushes harder, the force of the friction will then be enough to create a push.