The correct answer is C. constant.
Fickle means changeable, especially in one's affections or loyalties. The opposite of fickle is constant, which means unchanging or reliable.
A diseased person is ill, while a fast person is quick. A quick person is able to move or act quickly, while a fast person is able to travel or perform a task quickly.
Therefore, the only option that is the opposite of fickle is constant.
