Question

draw the free body diagram of a 100N wagon being pulled to the right on a rough, horizontal surface

Answer 1

You can also replace the box with a point for a FBD.

Answer 2

why wouldnt t be this

Answer 3

You have to add Fn (the normal force) and Fg (force of gravity) too, that should be obvious. Your graph should look like this then inclusive of those two.