The purpose of the twenty-one demands was to
a. demand control of german-held islands in the pacific
b. reduce china to the status of a japanese protectorate
c. demand that the british turn over control of hong kong
d. reduce korea to the status of a japanese protectorate

The purpose of the Twenty-One Demands was to B. Reduce China to the status of a Japanese Protectorate.

Basically, the 21 demands contains the following demands: 

*That the Japanese government would assume Germany's position in Kiaochow.
*That Manchuria and Mongolia be reserved to Japan for exploitation and colonization
*That Japan control the main coal deposits of China
*That the other powers be excluded from further territorial concessions
*That Japan guide China's military, commercial, and financial affairs.