Tann asked you to look for leads on the various missing arks.
You will receive this mission during the conversation with Director Tann at the end of Nexus Reunion mission.
Missing Arks
Mission objectives of Missing Arks
1. Find leads on the asari ark
This objective will be completed after the talk with Hydaria on Eos - objective "Take Cora to speak with Hydaria on Eos" of Cora Harper: Asari Ark mission.
2. Find leads on the turian ark
This objective will be completed when you read the email from Avitus Rix - objective "Check your email" of Turian Ark: Lost But Not Forgotten mission.
3. Find leads on the salarian ark
You will find this ark during "Locate the kett flagship in the Tafeno system" objective of Hunting the Archon mission.
