"Karina, the new student body president, was previously the class treasurer." is the sentence that is written correctly. Hence the answer is A. This is because the comma is correctly used to introduce the Dependent Clause.
A dependent clause is a statement that makes incomplete meaning all by itself except used in relation to the independent clause.
The dependent clause in the sentence above is:
"the new student body president", while the independent clause is
"Karina was previously the class treasurer."
See the link below for more about dependent clauses:
https://brainly.com/question/798413
