Final answer:
The active sentence 'They climb the mountain' in passive voice is 'The mountain is climbed by them.' The focus shifts from 'they' to 'the mountain.'
Step-by-step explanation:
The sentence 'They climb the mountain' converted into passive voice would be 'The mountain is climbed by them.'
In the passive voice construction, the object of the active sentence becomes the subject of the passive sentence, and the subject of the active sentence becomes the agent, typically introduced by the preposition 'by' after the verb.