Answer: B) Prologue.
Explanation: From the given options, the word that best completes the sentence, is the corresponding to option B: "We scurried into our theater seats just as the first line of the prologue was spoken, and I breathed a sigh of relief that we'd made it just in time," because the prologue is the introductory section or the first part of a literary, dramatic (a play), or musical work.