The passwd command is usually set suid root. this is because,
 

this is a Linux certification practice test question.

here are my choices.

A. non root users wouldn't be able to change there group membership

B. non root users wouldn't be able to change there group password

C. non root users wouldn't be able to change there password

D. non root users wouldn't be able to change there uid

whats the real answer.

Ignoring the suspect grammar the answer is C. The set suid allows any user to run the passwd command which will edit files owned by root. In this case the file containing the encrypted passwords - the /etc/shadow file I think.