The answer is: How did they trap the tiger?
The active voice is used when we wish to give importance to the subject who performs the action of the verb. In the active sentence above, the subject is they, who perform the action of the verb (trap the tiger).
On the other hand, the passive voice implies that the agent who carries out the action is not significant. In the example passive sentence, the person who traps the tiger does not really matter. What it actually intends to emphasize is the result of the action - that the tiger is under capture.