The Great Wall of China was primarily built to protect against which group?

(A) Japanese
(B) Mongols
(C) Tibetans
(D) Persians

The answer is: (B) Mongols
The Great Wall of China was originally constructed and extended over centuries to defend Chinese states and empires from nomadic invasions, especially from the Mongol tribes and other groups from the north, such as the Xiongnu.