This would strengthen the argument that there is no reason to believe the hypothesis, but it isn’t necessary. The professor argues against the hypothesis not because it wouldn’t explain the phenomenon, but because there is an alternate explanation for the phenomenon.

The argument is that there is no reason to believe the hypothesis. If (B) is false, then the hypothesis could be true without explaining the low visibility of star systems. But the stimulus still wouldn’t contain any reason to believe the hypothesis is true.

(C) says a hypothesis is likely to be correct if it explains a phenomenon that is not already explained. However, this doesn’t tell us that there is no reason to believe a hypothesis if a phenomenon it explains is already explained. It doesn’t matter what makes a hypothesis likely to be correct.

It doesn’t matter whether scientists who support the hypothesis accept the general theory of relativity — it only matters that in reality, it is able to completely explain the low visibility of stars. We don’t have to second-guess the information in the stimulus.

If the general theory of relativity depended on the “light-absorbing medium” hypothesis, then there would be a reason to believe the hypothesis — without the hypothesis the theory of relativity would fall apart and there wouldn’t be an explanation for the phenomenon.