The purpose of the Twenty-One Demands was to

ADemand control of German-held islands in the Pacific
BReduce China to the status of a Japanese protectorate
CDemand that the British turn over control of Hong Kong
DReduce Korea to the status of a Japanese protectorate

Verified Answer :

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.