When an object is at rest or moving with the constant velocity the net force on the object will be equal to zero.
When the crate is sliding down the ramp, definitely there will be a gravitational force acting on the crate.
But as the object is moving with the constant velocity, the net force acting on the crate is zero.
This implies that there is a frictional force acting on the crate balancing the downward gravitational force.
Thus the correct answer is option C, a frictional force is acting on it.
