The purpose of this structure is to protect from northern barbarians Option(b) is correct.
What is Great wall of China?
The Great wall of China was known to be as the series of the fortification which was actually built across northern borders in the period of the around 220 BCE.
It was built by Qin Shi Huang he was one to mastermind process of uniting walls into the one it was merely built to protect from the war of the northern invaders.
Therefore Option(b) is correct.
Learn more about Great wall of China here:
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