Question

As a crate slides down an inclined rough surface, its total mechanical energy

remains constant
might decrease or increase depending how large the slope angle is
might decrease or increase depending how large the friction coefficient is
increases
decreases?

Answer 1

As a crate slides down an inclined rough surface, its total mechanical energy decreases.
Since the surface of the incline is rough, friction is robbing mechanical energy from the crate all the way down, and blowing it away as heat into the surrounding air.