Cerberus Retribution Citadel mission can be completed after the first main mission of the second act.
To start Cerberus Retribution, you have to go to the C-Sec Outpost and near the office, you must find two arguing citizens.
They are located on the stairs that take you to the top floor of the Presidium Commons, in a corner, right next to the office.
When to start
After Priority: The Citadel 2
Citizens on the Citadel are preparing to carry out some sort of assassination at the C-Sec office in the Presidium Commons.
Go there and look for suspicious activity.
Credits, Experience, Paragon/Renegade Points
To complete the mission, go to the C-Sec Office, and on the couch to the left you will see Captain Aaron Sommers.
Talk to him and decide the prisoners fate, for up to 5 Paragon/Renegade Points.