Final answer:
The most indicative evidence of rigorous training for search and rescue dogs is the variety of challenging environmental conditions, such as different weather scenarios, they must adapt to during their training.
Step-by-step explanation:
In the context of search and rescue dog training, the statement that provides the best evidence that the training is rigorous would be "Trainers also work with dogs in a variety of conditions; rain, snow, and high winds all affect the scent trail." This statement highlights the diverse and challenging conditions under which training occurs, underscoring the intensity and complexity of the training process required for these animals to perform their duties effectively.
Other statements such as those referring to the dogs' innate abilities or the outcomes of their work, while important, do not directly convey the rigorous nature of training as clearly as the detailed description of their training conditions does. Rigorous training involves not just learning skills but adapting to a wide range of environmental factors that could impede the dogs' abilities to perform search and rescue tasks.