The best transition to connect the sentences and synthesize the information in the passage is:
A. In the 19th -and 20th -centuries, widespread efforts were made to reduce the number of wolves at Yellowstone National Park. As a result, grazing herds sharply rose.
This option indicates a cause-and-effect relationship, showing that the reduction of wolves directly led to an increase in grazing herds, effectively synthesizing the information from both sources.