Answer
Options A and B are inertial reference frames.
Work Step by Step
Option A states that the car is driving at a steady speed on a straight-level road. Therefore, using Newton's first law which states, "An object that is at rest will remain at rest, or an object that is moving, will continue to move in a straight line with constant velocity, if and only if the net force acting on the object is zero," we can conclude that A is an inertial reference frame (a reference frame where Newton's first law is valid) due to the object moving at a constant velocity in the same direction.
Option B states that the car is driving at a steady speed on a 10-degree incline. This would still apply to Newton's first law as the car is moving at a steady speed, meaning the net force is zero.
Option C would be wrong since the car speeds up after leaving a stop sign, meaning it is accelerating. Newton's first law states that for the net force to be zero, that must mean the acceleration of the object is also zero. Therefore, this case would not apply to Newton's first law, and would not be an inertial reference frame.
Option D also states that the car is driving at a steady speed, however, something moving at a steady speed does not qualify as an inertial reference frame. When going around a curve, you feel a force throwing you toward the side of the curve. This, along with the fact that when moving around a curve you are not maintaining the same direction, and therefore, acceleration is changing, is proof as to why D is not an inertial reference frame.
