Answer:
Your answer would be that the most suitable subordinating conjunction that correctly completes the sentence is the following one: until.
Step-by-step explanation:
The most suitable subordinating conjunction to complete the sentence above is until. A subordinating conjunction is a connecting word or phrase that introduces a dependent clause, joining it to a main clause. The subordinate conjunction in question means up to (the point in time or the event mentioned). In this case, the event that marks the end of Katrina's study session is she going to bed.