Atherosclerosis is a common cause, because there is a lack of blood flow in the person's body, and this can cause a male to be impotent.
Diabetes is also a common cause because once again it causes less blood flow.
In a younger man, a cause could be anxiety. This is because nervousness and being anxious can lessen blood flow.
If you have choices, I suggest choosing one that has to do with less blood flow happening, which that is the main cause.
I hope that this could possibly help you with your studies, and have an amazing day. If you have any more questions, send me a pm.