Final answer:
A family history of psychosis is the best-documented risk factor for delusional disorders, supported by studies showing a significant genetic component to these conditions.
Step-by-step explanation:
The best-documented risk factor for delusional disorder is A. Family history of psychosis. Research, including a 2004 study by Tienari and colleagues, indicates that genetics play a significant role in the likelihood of developing psychotic disorders such as schizophrenia. This study found that adoptees with a biological mother who had schizophrenia, especially when combined with a disrupted family environment, were much more likely to develop schizophrenia or a similar psychotic disorder than other groups. While other factors, such as a traumatic brain injury, substance abuse, and social isolation can also contribute to the development of delusional disorders, a family history, particularly of psychosis, is more significantly documented as a risk factor.