The correct answer is A. So
Step-by-step explanation:
In language, coordinating conjunction are short words that connect elements of the same importance. In the case of English language, there are only 7 words that are considered as coordinating conjunction and these are and, or, but, so, yet, for and nor. Considering this, from the options provided the only one that is a coordinating conjunction is the word "so" not only because this is one of the seven coordinating conjunctions in English but because indeed it links elements of the same importance as in the sentence "I was too tired, so I take a nap". Additionally, other words such as since or as are subordinating and not coordinating conjunctions.