The correct closing would be:- With warm wishes,
Answer: C) with warm wishes,
(on a side note, you may add your name for example:-
With warm wishes, Marie Hannah)
Use of subordinate clauses is more of a grammatical thing, and if the writer is not sure if a sentence is correct he/she may ask a peer reviewer to edit or comment on it. However, the body of the essay is a standard for every essay assigned and it should be much easier for the writer to tell if he/she is following the rules.
it would be c and d, both are trying to convince people
