Final answer:
The forces that act against the forward motion of a jet plane moving at a constant velocity are friction and air resistance, as they are the resistive forces opposing the thrust generated by the plane's engines.
Step-by-step explanation:
The question asks which forces act against the forward motion of a jet plane moving at constant velocity on a flat surface. When a jet plane is moving at a constant velocity, it means there is no acceleration and the forces are balanced. The thrust generated by the plane's engines is balanced by the resistive forces acting in the opposite direction. These resistive forces are friction and air resistance. Friction opposes the motion as the plane's wheels move on the runway, whereas air resistance, also known as drag, acts against the plane as it moves through the air. Gravity acts downwards, and the normal force from the ground supports the plane upwards, so these forces are vertical and do not oppose the forward motion when the plane is not accelerating vertically. Therefore, the correct answer is C. Friction and air resistance.