Which of the following reference points would allow you to observe the speed of a car that you are traveling in?
a. an airplane flying across the sky
b. a car traveling next to you at the same speed you are traveling
c. a car traveling next to you at a faster speed than you are traveling
d. a billboard on the side of the road.

The correct answer is option 'd' : a billboard on the side of a road.

Explanation:

We know that when we are in an moving object it seems to us that we are at rest and our surroundings are moving, this is the concept of relative motion.

We can observe speed of different things relative to other things weather they are moving or are at rest. In order to measure speed of an object we have to measure it with respect to a frame of reference and this frame is in which the observation of the speed shall be taken. This frame can be at rest or can be moving at any speed.

In our case

1) when we are travelling in the car if we observe the plane we will see the plane moving with a speed while it would not be possible to determine our own speed.

2) If we observe a car travelling next to us at the same speed we will observe it to be not moving at all or it shall be at rest with respect to us.

3) If we observe a car traveling next to you at a faster speed than we are travelling we shall observe that it goes past us but at a slower speed.

4) If we observe a billboard at the side of a road since it is at rest with respect to the ground we shall get an idea about how fast we are moving.

explanation:

newton's third law, for every action there is an equal and opposite reaction.