In the syndrome model of substance addiction, it is hypothesized that the reward center of the brain is not adequately stimulated by usual activities, so the individual turns to drugs to stimulate this pleasure pathway.

The answer to this question would be: C. deficiency 

In reward deficiency syndrome theory, the individual is having a deficiency in the reward system and that causes them to seeks drug to increase the reward. The behavior could be different in other individuals, varying from addictive behavior, compulsive behavior or impulsive behavior.