The correct answer is B, C, and D. OCD, or Obsessive-Compulsive Disorder, is believed to be caused by an imbalance in serotonin, a neurotransmitter in the brain. Additionally, research suggests that a malfunction in the basal ganglia, a group of structures involved in movement and behavior regulation, may contribute to the development of OCD. Furthermore, overactivity in the worry center of the brain, known as the prefrontal cortex, has also been associated with OCD. Therefore, a combination of these factors (B, C, and D) can cause OCD.