Answer: C) subordinating conjunction.
Explanation: A conjunction is a word that joins two clauses, a subordinating conjunction is a conjunction that is used at the beginning of a subordinate clause (a phrase that doesn't have complete meaning on its own), to join it with an independent clause (a phrase that has complete meaning on its own). In the given sentence "until" is acting as a subordinating conjunction that joins the independent clause "I couldn’t understand the jokes" and the subordinate clause "Until I had seen the show myself."
