No sooner had we passed the cave than…

The phrase “No sooner had we…” is used to indicate that one event happened immediately after another, emphasising quick succession. It is typically followed by “than” and then the second action. Of the options given, the sentence is best completed as:  “No sooner had we passed the cave than a sudden roar emerged from within”. This is both grammatically correct and contextually meaningful. The other options either break the sequence of immediate events or do not logically follow the cause-and-effect or time pattern implied by “no sooner… than.”