B. He tried everything to keep the shelter open—he even begged his friends for support.
Explanation:
Dashes can be used to introduce a pause, an explanation or an interruption in a sentence, and are more informal than using parenthesis. In that respect, the dash in the sentence given precedes a clarification of the first clause. Thus, it demonstrates evidence of how the person tried to keep the shelter open.
The rest of the options contain incorrect uses of dashes, since they cannot separate clauses in conditional sentences or replace commas setting items apart in a list.
