The correct answer is D. who was elected last week.
A dependent clause, also known as a subordinate clause, is a type of a clause that cannot exist or stand on its own - it has to be part of a larger, independent clause in order to make sense. Without the independent clause, the dependent clause is incomplete. This is the case with option D - when you just say who was elected last week, it doesn't make much sense unless you complete the clause.
