This best captures the meaning of the phrase, because (A) describes the long-existing evidence that demonstrates Meitner’s insight. Meitner’s insight was that neutron bombardment involved splitting uranium atoms. These experiments had been occuring since 1934.

This is too narrow, because the author asserts in the last line that the relevant evidence “had been present for some time, lacking mainly the right conceptual link.” By this phrase, the author indicates that the relevant data had been around for a relatively long time. This connects back to the author’s description in P1 of neutron bombardment experiments since 1934. So it wouldn’t make sense for “the relevant evidence” to refer to Meitner’s own experiments in 1938. This would ignore the experiments described in P1 and would not be a particularly long time before Meitner’s insight.

This is too narrow, because the author asserts in the last line that the relevant evidence “had been present for some time, lacking mainly the right conceptual link.” By this phrase, the author indicates that the relevant data had been around for a relatively long time. This connects back to the author’s description in P1 of neutron bombardment experiments since 1934. So it wouldn’t make sense for “the relevant evidence” to refer to Hahn’s experiment. This would ignore the experiments described in P1 and would not be a particularly long time before Meitner’s insight.

This is too narrow, because the author asserts in the last line that the relevant evidence “had been present for some time, lacking mainly the right conceptual link.” By this phrase, the author indicates that the relevant data had been around for a relatively long time. This connects back to the author’s description in P1 of neutron bombardment experiments since 1934. So it wouldn’t make sense for “the relevant evidence” to refer some narrow aspect of Hahn’s experiment, as (D) describes. This would ignore the experiments described in P1 and would not be a particularly long time before Meitner’s insight.

The “relevant evidence” would, at a minimum, be evidence that supports Meitner’s insight that neutron bombardment involved splitting the uranium atoms. So what matters is that uranium could be split from neutron bombardment. But the lack of identification of the chemical products produced by neutron bombardment isn’t evidence that atoms can be split. In any case, (E) also ignores the relationship between “the relevant evidence” and the experiments described in P1. The evidence described in the last line “had been present for some time,” so it must refer to the long-existing evidence of atom-splitting.