The forces acting on a child sitting in a seat are described by the free-body diagram shown. What is the net force acting on the child? –100 N 0 N 100 N 200 N

The net force is 0 N.
To find the total net force, all you need to do is add the forces.
So 100 + -100 = 0
Most of these questions are trick questions, but if you know Newton's laws of forces, then you should know that all forces should cancel out with each other.
Hope this helps!!!