Aperson who has an insatiable craving for any substance,
a. is addicted.
b. should redirect their thoughts into other, more pleasing areas.
c. is generally weak and not in control of their life.

A person who has an insatiable craving for any substance A. is addicted. Regardless of the fact whether you cannot stop using drugs, alcohol, cigars, coffee, or the Internet, even, you are still addicted, the object of your addiction is not as important. If you cannot make yourself stay away from all of these, it means that you may need help dealing with your addiction. B is not correct because usually this won't help you fight your addiction - it is just you running away from your problem. When it comes to C, it is also not necessarily correct, because many strong people who have control over their lives have also become addicted to something over the course of their lives.

Objective pronoun is the answer.