Concept:The fixed correlative structure "no sooner ... than" links two events that happen immediately one after the other.
Explanation:The word "no sooner" is always paired with "than" in standard English.
This pairing shows that the second action occurred right after the first action.
In this sentence, "they arrived" is the first event, and "we left the hall" is the second event that happened immediately afterward.
Notice the correct structure uses the past perfect with inversion: "No sooner had they arrived than we left the hall."
The words "when", "so", and "therefore" do not form this required grammatical pair.
Therefore, only "than" correctly completes the sentence.
Answer:A. than