Answer:
Letter A is the correct option.
Step-by-step explanation:
A nonrestrictive clause is understood as an adjective clause that does not give essential information, and because of that, the meaning of the sentence remains the same with or without this clause. The first option has the nonrestrictive clause set off by dashes and not by commas. Also, the meaning of the sentence would not be changed if the clause were not there. In the second sentence, the dash is used to give a different sense to the sentence, indicating the second action to be a little distant from the first one. In the third option, the clause seems to be in parentheses (for all the students) and the meaning of the sentence would change if that clause were not marked as it is. Last option is not correct because nonrestrictive clauses are usually indicated with commas or dashes (as in the first sentence) and this last option lacks these punctuation marks.