Correct Option (D)
According to Newton's First Law of Motion, an object in uniform motion continues in that state unless acted upon by an external net force. Alternatively, from Newton's Second Law of Motion, the net force (F) on a body is given by F = ma, where m is the mass and a is the acceleration. Since the car is moving at a uniform speed of 60 km/h, its acceleration (a) is zero. Therefore, the net resultant force (F) on the car is F = m * 0 = 0. This implies that all individual forces acting on the car, such as the driving force and resistive forces (e.g., friction, air drag), are perfectly balanced.
Incorrect Options:
1) driving force in the direction of car's motion: While a driving force is exerted by the engine, for the car to maintain a uniform speed, this force must be precisely balanced by the opposing resistive forces (e.g., friction, air resistance). Thus, the net force is not solely the driving force but the sum of all forces, which is zero.
2) resistance force opposite to the direction of car's motion: Resistive forces like friction and air drag are indeed present and act opposite to the direction of motion. However, for uniform speed, these forces are balanced by the driving force, resulting in a net force of zero, not just the resistance force.
3) an inclined force: On a level road, the primary forces acting horizontally are the driving force and resistive forces. There is no inherent inclined force component that would represent the net resultant force for a car moving at a uniform speed on a flat surface.