As the skydiver is falling, she is pulled down by the force of gravity, which tends to accelerate her towards the earth, while at the same time the drag force Fd opposes this motion. When she first jumps out of the plane, her speed downward is not very big, and so the drag force is small. As gravity accelerates her downward, her speed increases, and so does the size of the drag force. As the drag force increases, it opposes the force of gravity, decreasing the size of the acceleration. At some point, the drag force will increase to be the same size as the force of gravity. At that point, the skydiver will stop accelerating and hit a constant velocity, known as the terminal velocity.
(b) When the parachute is opened, the b coefficient in the drag increases, and the drag force will be higher than the weight force. This will cause an upward acceleration that will have the effect of reducing the downward velocity. As the velocity decreases, the drag force will decrease, until the drag force once again balances the weight force at a new, lower terminal velocity, hopefully leading a safe landing!
Einstein argues that if the principle of relativity weren't valid, then the laws of physics would change depending on the reference frame. Different frames would have different laws and presumably the laws in some frames would be more complicated while in other frames the laws would be simpler. From all the different possible references frames/laws of physics, you should be able to select one that has the least complicated laws of physics. In that case, you'd be justified in calling this reference frame the "best" or "at rest" frame, and all the other reference frames would build on that.
If such a "best" frame exists, the earth is certainly not in it continuously. The motion of the earth is constantly changing as the earth spins on its axis, orbits the sun, and moves with the solar system around the center of the galaxy. Sometimes the earth might be at rest with respect to the "best" frame, but at other times it wouldn't be. So, if the principle of relativity were not true, we should see changes in the laws of physics over time. These changes should be observable to us (at least after careful measurements) and therefore, if the principle of relativity were wrong, we should already know it.
In Boltzmann's version, for our current universe to arise as a statistical fluctuation from a higher entropy state, the most likely scenario is that it arose just moments ago. This is because this would entail the smallest deviation from a high entropy state. If it happened any further back in time than just instants ago, it would require an even larger fluctuation, since, from our memories and reasoning, entropy was lower further back in time. This means all our memories are fabricated as part of this fluctuation, making our experience of reality completely unreliable. In contrast, inflationary cosmology says a tiny fluctuation in entropy in the early universe (the inflaton field getting caught in a small region a space, drives the creation of the entire, higher entropy universe today. This theory is nice because it doesn't destroy our ability to rely on the memories and reasoning that got us to this point in the first place.