Final answer:
Both the Roman and Han empires were frequently attacked by smaller groups living near their borders, a similarity encapsulated by option D.
Step-by-step explanation:
The similarity between the external threats that faced both the Roman and Han empires is described by both empires were frequently attacked by smaller groups living near their borders. This chronic situation is well-documented historically, with both empires contending with the pressures and incursions from various nomadic and barbarian tribes over extended periods of time.
The Romans faced significant challenges from Germanic tribes like the Goths and Vandals, while the Han Chinese grappled with nomadic tribes such as the Xiongnu, to the point that the Han even built structures like the Great Wall to protect against these constant threats.