The Right Answer

This one is a straight Math Strategy question. The colon indicates that the idea on the right should be a continuation of the idea on the left. On the right-hand side, we can see that it says "most of them knew very little about foreign countries". The blank (which also describes the same people) should be conveying this idea, something like "narrow-minded" or "parochial"

The best match for this blank is insular.

The Wrong Answers

partisan: This choice has no evidence. There is no evidence that the people were biased in any way.
erudite: This choice contradicts the logic of the sentence. The sentence says the people "knew very little", not "knew a lot"
cosmopolitan: This choice contradicts the logic of the sentence. If they were "cosmopolitan", they would be well aware of other countries
imperturbable: This choice has no evidence. There is no evidence about how calm the people were.