This explains why scientists hadn’t considered the possibility of a fifth force, but it doesn't tell us whether the fifth force actually exists or not. Whether or not scientists were thinking about the fifth force before a certain time period doesn't in itself make the fifth force more or less likely to exist.

If this answer choice is true, it strengthens the argument by getting rid of a potential weakeness -- that a fifth force might contradict some other aspect of settled science. Since it doesn't contradict other known results, the fifth force seems more likely to exist.

This would weaken the hypothesis by suggesting that the fifth force isn't its own force at all, just an aspect of another force.

This would weaken the hypothesis by calling into question the experiments used to hypothesize the fifth force and making those results seem unreliable.

Whether or not other discoveries in physics were happening is irrelevant to whether or not the fifth force actually exists or not. The hypothesis could still be false even if other legitimate theories were being developed at the same time.