I rushed through this last question and got it wrong. The opening sentence for the third paragraph cleary states it's purpose: to explain the "reasons for the effects."
The key to getting this question correct and doing it quickly is understanding what "these effects " refers to. While reading the passage one should ask what effects refers to and push back to the previous paragraph. This will help with understanding the overall structure of the passage. It's clear that the previous text is referring to the broad coalition within the Korean American community and society at large. Therefore, the "effects" refers to this broad coalition.
Also, in the first paragraph the author more or less outlines part of the passage. We know there was an unprecedented coalition and also that there was a deep impact on the Korean American community. In the second paragraph the author explains who participated in the coalition and it's effects but we can expect an explanation of why/how this unprecedented coalition formed. Why was it unprecedented? This paragraph explains why.