Concept:The expression "But for" introduces a past unreal condition, meaning the situation did not happen because of an intervention.
Explanation:"But for the timely intervention of the police" means "If the police had not intervened."
This points to a hypothetical past event that did not actually occur.
For unreal past conditions, use "would have been" plus the past participle.
The thief escaped the action because the police stopped it.
Therefore, the correct completion is "would have been lynched."
Answer:B. would have been lynched