The answer is C) The country had a low population density and lacked any defense system.
The ease with which European imperialists conquered China was due to its sparse population and absence of defense mechanisms.
During that time, China did not possess a formal or adequately trained military.
These aspects made it easier for invaders to penetrate Chinese lands, despite the challenging geography which included mountainous regions that had kept China isolated for many years. The Mongols invaded and controlled significant parts of China in the 1120s AD, and another instance of invasion was the Japanese occupation between 1937 and 1945.