Combine the two sentences using a suitable adverb/adverbial phrase:
You should call the restaurant and make a reservation; _____ you might not get a table. [Intermediate]
A contrasting conjunctive adverb is used to connect two clauses which show opposing function, here “you...reservation” and “you...table”, and hence Option C is correct. The rest of the options do not connect clauses in contrast, hence incorrect.