Final answer:
In NASCAR, when the green flag waves after a crash, it signifies that the race restarts, allowing drivers to resume racing at competitive speeds.
Step-by-step explanation:
When the green flag waves in NASCAR after a crash, it signifies that the race has officially restarted. This is a signal for the drivers that the track is clear, and they can return to racing at competitive speeds. The waving green flag indicates the end of a caution period, and therefore, option A) The race restarts, is the correct answer. During this time, caution laps are no longer counted, and while pit stops are allowed, they're not specifically related to the waving of the green flag. Therefore, option C) Pit stops are allowed is not directly correlated with the green flag. Additionally, option D) Drivers switch positions is incorrect because the positions are maintained from before the caution period, unless drivers choose to pit, which can alter positions due to varying pit strategies and times.