Final answer:
Manifest Destiny is the concept that best exemplifies John O'Sullivan's statement in 1845. Therefore, the correct option is b. Manifest Destiny.
Step-by-step explanation:
The concept that best exemplifies John O'Sullivan's statement in 1845 is Manifest Destiny. Manifest Destiny is the belief that the United States was destined and ordained by God to expand its democratic institutions across the North American continent. This expansion would involve spreading American culture, thoughts, and customs to the native inhabitants and developing the land.