What aspect of mobility should be observed when performing a physical examination?

When performing a physical examination on a dog, observing the ability to walk, run, and jump normally is crucial for assessing overall mobility. These activities are fundamental to a dog's day-to-day functioning and provide insight into the health and integrity of the musculoskeletal system. The ability to execute these actions reflects the joint function, muscular strength, balance, coordination, and overall physical fitness of the animal.

While the other aspects mentioned in the options—such as sitting, reactions to new environments, and interest in play—are important for a comprehensive behavioral assessment, they do not directly assess mobility in the same way. Sampling mobility through dynamic actions like walking, running, and jumping gives clearer indications of potential issues such as lameness, pain, or restrictions that may not be visible when the dog is simply sitting or exhibiting behavioral responses to stimuli. This focus on comprehensive movement helps ensure that any underlying issues affecting mobility are identified and addressed appropriately.