The correct option is C You must leave early. You will get late for your interview.
A compound sentence is made up of two independent clauses, which are
complete sentences on their own. Alternative compound sentences express a choice between two alternatives. Option B presents two alternate ideas, where one is the possible outcome of the other, hence 'otherwise' can connect these together and option B is correct. Option A does not present two alternate ideas, but states the events "snow...fall" and "roads...blocked" that happened one after the other, the latter being the result. Hence A is incorrect. Option C expresses two contrasting ideas and Option D contains two unrelated statements, that do not influence one another. Hence C and D are incorrect.