Final answer:
Napoleon's greatest enemy during his reign was Great Britain, who consistently opposed his ambitions and dominated the seas, thwarting his plans for invasion. The option (D) is correct.
Step-by-step explanation:
The country that remained Napoleon's greatest enemy during his reign as emperor was Great Britain (D). Despite his formidable military successes on the continent and his establishment of a large empire, Napoleon was unable to conquer Great Britain, which continued to dominate the seas with its superior navy.
While Napoleon did forge a temporary peace treaty with Great Britain and even momentarily allied with Russia, it was always Great Britain that posed the most consistent and unyielding opposition to his ambitions, especially after the decisive Battle of Trafalgar in 1805 where the British navy dealt a significant blow to Napoleon's hopes of invading Britain. Therefore, option (D) is correct.