If citizens do not exercise their right to vote, then democratic institutions will crumble and, as a consequence, much valuable social cohesion will be lost.
Conclusion
(implicitly) Citizens should vote.
Evidence
If citizens don't vote, democratic institutions will crumble and lots of social cohesion will be lost. Yes, one vote isn't going to make a perceptible difference, but you have to think about the act of non-voting on a large scale. If it would be socially damaging for large numbers to not vote, then it would be wrong for an individual to not vote. (Just as if it would be bad for society if large numbers of people engaged in theft, then it's wrong for an individual to engage in theft).
Evaluate
Usually, on Main Conclusion questions, the conclusion is found in the first sentence or as part of some "but / yet / however" style rebuttal. Here we kind of have neither. The first sentence isn't a supported idea. The argument doesn't say.
The argument establishes in that first sentence that there are negative consequences if citizens don't vote. Then it presents a discussion that is sort of rebutting the suggested position of, .
"Yes it does!", the author thinks. Why does it matter? Here, we get supporting ideas: - if collective omission would damage social cohesion (which we know from the first sentence it would), then individual omission is wrong.
Goal
Since it's harder to land on one explicit claim to call the Conclusion, I would look for something resembling , but ultimately be flexible and test each answer by the standard of ... 1. Does this sound like an opinion the author holds? 2. Did she support this opinion?