Final answer:
The sentence uses a coordinating conjunction, which is 'so', to connect two independent clauses of equal grammatical importance.
Therefore, option D is correct.
Step-by-step explanation:
The sentence "I received my grades today, so my friends took me out to celebrate" uses a coordinating conjunction. A coordinating conjunction connects words, phrases, or clauses that are of equal grammatical importance. In this case, 'so' is the coordinating conjunction connecting two independent clauses. There are seven coordinating conjunctions in English, often remembered by the acronym FANBOYS: for, and, nor, but, or, yet, and so. Coordinating conjunctions are used to connect words, phrases, or clauses that are of equal importance. In this sentence, the coordinating conjunction 'so' connects the main clause 'I received my grades today' with the dependent clause 'my friends took me out to celebrate'.