Final answer:
The statement that the frictional force is zero while pulling a crate up a rough inclined plane at a constant velocity is not true; friction is present and it opposes the motion, balancing the force applied by the girl.
Step-by-step explanation:
The question is about a physical situation where a girl is pulling a crate up a rough inclined plane at a constant velocity. Among the given statements, the one that is not true is (d) The frictional force is zero. When pulling a crate up a rough incline, there is always some amount of friction opposing the motion. Since the crate is moving at a constant velocity, this implies that there is a balance of forces; the force applied by the girl is equalized by the combined effect of friction and gravity.
Choices (a), (b), and (c) could be considered true given the situation. The work done by the girl is indeed equal to the force she applies times the displacement (a). The gravitational force does work on the crate when it is being lifted against gravity (b). And there is no net force acting on the crate in the horizontal direction if the velocity is constant, meaning all the horizontal forces are balanced (c).
In summary, frictional force is present whenever there is motion on a rough surface, and in this scenario, it cannot be zero because it must oppose the motion and balance the forces to result in a constant velocity.