Final answer:
The earliest Chinese civilizations in the North China Plain faced frequent threats from nomadic tribes and were not typically safe from foreign attacks.
Step-by-step explanation:
The earliest Chinese civilizations in the North China Plain were not usually safe from foreign attacks. Despite the availability of fertile lands for agriculture, communities in these areas were often threatened by the nomads from the north and the west. The nomadic tribes, skilled in mounted horseback riding and archery and with access to advanced military technology, could easily raid Chinese farming settlements due to the flat terrain and lack of natural barriers. As a result, the imperial Chinese authorities found it challenging to find a satisfactory solution to these invasions, leading to periods when China experienced incursions and even dynastic changes, like the Yuan and Qing. Hence, the correct answer is D. usually safe from foreign attacks.