The correct options are
A They may have stood up against an act of discrimination that they faced.
B They may be well-respected because they treat everyone with dignity and are thus called upon to resolve community issues.
C They may have stood up against an act of discrimination which they witnessed.
This is because they may have stood up against an act of discrimination that they faced or which they witnessed. Or they may be well-respected because they treat all persons with dignity and are, therefore, trusted and called upon to resolve issues in the community.