Final answer:
The best use of a coordinating conjunction is found in sentence A, where 'so' appropriately connects two independent clauses, making the sentence clear and effective.
Step-by-step explanation:
The sentence that shows the BEST use of a coordinating conjunction is A) She paid for the extra ticket, so please pay her back. This sentence correctly employs the coordinating conjunction 'so' to connect two independent clauses, which could stand as sentences on their own. Coordinating conjunctions include words like 'for', 'and', 'nor', 'but', 'or', 'yet', and 'so' (often remembered through the acronym FANBOYS), and they are used to connect words, phrases, or independent clauses that are of equal syntactic importance. The correct use of a coordinating conjunction helps in providing clear and concise communication.