Final answer:
If a plague killed all of the American toads in a region, the population of Eastern hognose snakes would likely decline.
Step-by-step explanation:
The most likely outcome if a plague killed all of the American toads in the region would be that the population of Eastern hognose snakes would decline (option B).
This is because American toads are the major prey species for the Eastern hognose snakes, and without them, the snakes would not have enough food to sustain their population. As a result, the snake population would decrease.