What advice would you give her to improve her diagram? Choose all that
apply.

A, C

Explanation:

In order for a box to be moving at constant velocity, that means acceleration must be 0(as the speed isn't increasing). Thus, since F = ma, and a is 0, the net force must also be 0. The diagram must be completely balanced in every direction, which is why the two horizontal arrows must be equal values.

C is also true because gravity is a stronger force here than the applied force. We can calculate the force due to gravity with Fgravity = mg. Thus, we get the force as a result of gravity = 9.8*50, which is around 500N and much larger than the applied force of 200N.

the one with the highest speed

in physics zero. in muscle fatigue could be high