Some of the early attempts to apply eugenics in the human realm were based more on ________ than on understanding.

“Than” sets up a contrast between the blank and understanding. Of all these options, 'enthusiasm' and 'erudition' (learning, knowledge) seem to fit the bill. But on close scrutiny, erudition would be in tandem with understanding, it would not fit the contrast, whereas “enthusiasm” maintains the contrasting prediction, thus replaces the blank. 'Radicalism' is the quality or state of being radical, esp. in politics which would be off the track, hence ruled out. Hence, (2) is the answer.