Consider an object sliding at constant velocity along a frictionless surface. which of the following best describes the forces on the object?

there is a net force at right angles to the direction of motion

there is a net force in the direction of motion, that is independent of the object’s velocity

there is a net force in the direction of motion, that depends on the object’s velocity

there is zero net force on the object

-- The net vertical force on the object is zero.
Otherwise it would be accelerating up or down.

-- The net horizontal force on the object is zero.
Otherwise it would be accelerating horizontally,
that is, its 'velocity' would not be constant.  That
would contradict information given in the question.

The total net force on the object is the resultant of the
net vertical component and net horizontal component.

Total net force =  √(0² + 0²)

                         =  √(0 + 0)

                         =  √0

                         =  Zero.

The correct answer is the last choice on the list.

Also, you know what ! ?  It doesn't even matter whether the surface it's
sliding on is frictionless or not. 

If the object's velocity is constant, then the NET force on it must be zero. 
If it's sliding on sandpaper, then something must be pushing it with constant
force, to balance the friction force, and make the net force zero.  If the total
net force isn't zero, then the object would have to be accelerating ... either
its speed, or its direction, or both, would have to be changing.