I would like to add my 2 cents explanation so that I can get a clear understanding.
The argument has a very nuanced difference, which is really hard to notice. We just know that if there is dependence, it is not necessary that there is abuse, which in turn means that it is not necessary that it can be defined as addiction. The author provides an example, and all options are related to this example. So the answer should come from within the example itself. Now, let’s try to negate within the scope of the argument — then we can understand it more clearly.
The cancer patients can become dependent on morphine to relieve their pain, but this is not abusing the drug.
Respecting the boundary of this example, we can infer that a cancer patient can become dependent (it is allowed to become dependent), but it is not necessary that the patient is abusing the drug. This means the cancer patient is addicted — according to the argument. In other words, before giving the example, the author assumed that the cancer patient is addicted and can become dependent on the drug, but it doesn’t come under the category of abuse.
Negating option C would make the author very ashamed, because if, in the first place, the cancer patient is not addicted, then why create such a mess by claiming that addiction doesn’t come with dependence and abuse? One can argue, as we GMAT aspirants already know, that life is complex enough — why make it even more complicated? Therefore, the cancer patient must be addicted. And again, looking at the conclusion, I observe that there is no mention anywhere that the cancer patient is addicted. I mean, we are very busy in life, and we make this subtle assumption — and we fail to detect it.
The author is not claiming a very extreme conclusion. If we are able to find a case where there is dependence on a drug but no abuse, we can strengthen the author’s conclusion. But in the very first place, for the example of the cancer patient the author has taken, the patient must be addicted.
Btw, I loved this question — it was making me nuts and keeping me excited the whole time. Again, my mind is buzzing — love the way you lie in the background.
AV