Define obsessive-compulsive disorder (OCD).

Obsessive-Compulsive Disorder (OCD) is characterized by the presence of unwanted, intrusive thoughts (obsessions) and repetitive behaviors or mental acts (compulsions) that an individual feels driven to perform in response to the obsessions. These obsessions often cause significant anxiety or distress, leading the individual to engage in compulsive behaviors as a way to mitigate that anxiety or prevent a feared outcome. The essence of OCD lies in this cycle of obsessions and compulsions, which can significantly impair daily functioning and quality of life.

In contrast to the other options, which describe different psychological issues, option B accurately captures the core components of OCD. Constant sadness aligns more with depressive disorders, developmental disorders pertain to conditions like autism spectrum disorder, and personality disorders involve enduring patterns of thinking and behavior that deviate from cultural expectations. Thus, option B succinctly encompasses the critical features of OCD, making it the correct choice.