This is a cause and effect argument. The observed cause that the experimenters are trying to explain is "a measurable amount of helium-4 gas" found in the air of the chamber after the experiment. The experimenters' proposed cause is "that fusion had been achieved."
We are asked to weaken the experimenters' conclusion so let's take a look at the answer choices:
(A) is incorrect because it is completely irrelevant. We are trying to cast doubt on the explanation for the measurable amount of helium-4 that was present after the experiment. We do not care about the presence of other gases.
(B) is incorrect because it is completely irrelevant. We do not know whether tritium and gamma rays were also present.
(C) is CORRECT because it weakens the experimenters' conclusion by pointing out a possible alternate cause. The presence of helium-4 in the chamber after the experiment wasn't because fusion was actually achieved but rather because helium-4 is normally found in ordinary air.
(D) is incorrect because it is completely irrelevant. We do not care what happens to the helium-4 later on. We are simply trying to explain why it was present in the chamber after the experiment.
(E) is incorrect because we do not know whether large amounts of heat were released during this experiment or not.
Hope this helps! Please let us know if you have any other questions.
