Which of these is a possible cause of depression? a. surviving a traumatic event or living in a harmful environment b. being confident in situations that make you feel anxious c. not experiencing mood changes d. having a good diet and exercise regime

I believe the best option is A, Surviving a traumatic event or living in a harmful environment. Being confident and being healthy are good things in life, while if you have something extremely bad happen to you it can lead to depression. Mood changes are a common thing for most people, like happiness, sadness, being angry, etc. 

Hope this helps!

A.surviving a traumatic event or living in a harmful environment. 
i think. :)