Officer Stern Is the Internal Affairs Investigator at a Correctional

Officer Stern is the internal affairs investigator at a correctional institution. He is currently investigating 5 correctional officers for behaving in a less than professional manner. Officer Stern loves his job because of the daily challenges.
-Which case will Warden Stern not investigate?

A) personal dealings between staff and inmates
B) allegations of physical abuse of inmates by staff
C) inmate-on-inmate assault
D) he will investigate all of these