Final answer:
The rubber stopper pops out of the test tube in a vacuum chamber because the pressure inside the test tube increases when the air is pumped out, creating a pressure differential that forces the stopper out.
Step-by-step explanation:
When a test tube with a stopper is placed in a vacuum chamber and the molecules are removed from the chamber, the rubber stopper pops out of the test tube because the pressure inside the test tube increases. During this process, as air molecules are pumped out of the vacuum chamber, the pressure outside the test tube decreases. Inside the test tube, the pressure remains higher compared to the pressure outside in the evacuated space, creating a pressure differential. This difference in pressure forces the stopper out of the test tube, as the gas inside the test tube pushes outwards. In this case, the correct answer is 2) The pressure inside the test tube increases.