Final answer:
The passive form of the given sentence is 'Horses and mules are used to carry their baggage.' This transforms the original subjects (horses and mules) into the focus of the sentence while de-emphasizing who performs the action.
Step-by-step explanation:
The passive form of the sentence "They use horses and mules to carry their baggage" would be "Horses and mules are used to carry their baggage." In constructing this passive sentence, the object (horses and mules) of the active sentence becomes the subject of the passive sentence. The use of passive voice allows the sentence to emphasize the action (the carrying of baggage) and the subjects (horses and mules) rather than the doers of the action.