Which dynasty built protective walls along the northern border that will later be rebuilt and connected to form the great wall of china

The Qin dynasty was the first dynasty to start connecting the various walls into a greater whole. That happened around 221 to 206 BC and was done under the reign of Qin Shi Huang. Though the wall was later improved still by other dynasties, it was during the Qin dynasty that the Great Wall of China was first formed into a cohesive structure.

c would be the correct !

hope this !

~bbgluver