Answer: Option (D) is the correct answer.
Step-by-step explanation:
When a dog runs faster then due to more force applied by the foot there occurs large footprints of the dog.
Spaces between the feet of dog increases due to running faster as running makes an individual to take long steps.
Whereas when a dog walks normally then his footprints will be smaller in size.
Therefore, we can conclude that the BEST alternative to Rob's conclusion is that one of the dogs ran faster than the other.