From the research on unipolar depression we learn that: a) clinicians know very little about what contributes to the development of the disorder. b) unipolar depression is clearly solely the result of biological factors. c) clinicians have a clear understanding of the cause of the disorder. d) it is still not entirely clear how all of the identified biological, psychological, and sociocultural factors are related to unipolar depression.

The correct answer is letter "C": clinicians have a clear understanding of the cause of the disorder.

Explanation:

Unipolar depression or major depressive disorder (MDD) is the emotional state in which sadness and/or lack of interest controls someone's behavior. Researches state that the most influential factor that provokes depression is in the individual's family tree: genetics. But some of the symptoms of the very same disorder: excess or lack of appetite or not setting a sleeping schedule may trigger depression.