Concept:The phrase "no sooner ... than" is a fixed correlative conjunction used to show that one event happens immediately after another.
Explanation:When a sentence begins with "No sooner had...", the second part must be introduced by "than".
This is because "no sooner" is always paired with "than", never with "when", "and", or "then".
The structure describes two past events happening in quick succession.
Therefore, the correct word to complete the sentence is "than".
Answer:No sooner had the judge read the verdict
than pandemonium broke out.