A. Karina, the new student body president, was previously the class treasurer.
Explanation:
The structure "the new student body president" functions as an appositive in the sentence. That means its goal is to give further information or an explanation about something that was just mentioned - in this case, about Karina. We now know that Karina is the new student body president. Appositives should have commas immediately before and after them. Those commas help establish that this in an extra piece of information included in the sentence. With that in mind, we can see that letter A is the best option:
A. Karina, the new student body president, was previously the class treasurer.
