Final answer:
When a car is cruising at a constant speed, the net external force on it is zero. Therefore, the force of friction is equal to the force applied by the engine, which in this case is 75.0 N.
Step-by-step explanation:
If a car is cruising on a highway at a constant speed of 30.0 m/s and the engine is applying a force of 75.0 N, we can determine the force of friction using Newton's first law of motion, which states that an object in motion will remain in motion with a constant velocity unless acted upon by a net external force. Since the car is cruising at a constant speed, this indicates that the net external force acting on it is zero. Therefore, the force of friction acting against the engine's force must be equal in magnitude and opposite in direction to maintain the constant velocity.
Thus, the force of friction would also be 75.0 N, which is option B. We don't need to use the given acceleration due to gravity (g = 10 m/s²) because it doesn't factor into this scenario where the speed is constant and there is no vertical motion involved.