This wouldn't weaken the experimenters' conclusion. We already know the chamber was filled with "air" to begin with, which suggests the presence of gases that aren't helium-4. In any case, the experimenters' conclusion doesn't require helium-4 to have been the "only" gas found, so this doesn't weaken their argument.

This doesn't weaken the argument. We're never told that these other by-products, tritium and gamma rays, are not present. We're just told that one by-product of fusion, helium-4, is present. To undermine this argument, we'd want another explanation for the presence of helium-4 that doesn't involve nuclear fusion. This answer choice doesn't give us that.

This would weaken the argument. We know the chamber was filled with "air" to begin with, so this answer choice suggests that the helium-4 in the chamber could have been there from the start, and not a by-product of nuclear fusion. This effectively weakens the experimenters' argument.

This doesn't weaken the argument. This just tells us that, in order to have detected any helium-4 gas at all, the experimenters most likely measured the helium-4 level soon after the experiment, not several hours later. But that doesn't explain why the helium-4 is there in the first place, let alone provide an alternative explanation besides nuclear fusion.

This would only weaken the experimenters' argument if we were explicitly told that a large amount of heat was not produced. Since we're not told this, we still have to explain the presence of helium-4, a potential by-product of fusion. This answer choice doesn't suggest an alternative explanation to the experimenters'.