Full Synchronisation Requimement - Pickpocket the key to the cell.
Once you've spoken to the prisoner through the bars of his cage, you'll learn that you need to obtain the key from a nearby guard. However, this guard is patrolling in a restricted area, meaning that you're going to have to be stealthy if you want to get that key and full synchronisation without any mishaps. First things first, getting close to the guard in question without being detected can be a little tricky. You can take the acrobatic approach, and scale a nearby building (keep an eye out for rooftop Templar Riflemen, they're very aggressive and eager to take a shot at you!) to jump over the wall, or alternatively you can hire a group of followers to distract the guards blocking the entrance to the restricted area. If you have any Cherry Bombs or similar distractionary items left, those could also be used to draw the guards away, getting you into the area without a fight.
When you reach the guard in question, fast walk behind him and pickpocket him as you did earlier when trying to raise funds for the armour upgrade. Hopefully he won't notice you and you can make a quick getaway over the wall and back to the prisoner's cage. If you however happen to get in a fight with the guard and the others in the area, all is not lost. You can use the new counter steal technique to pickpocket him in combat, and then finish them off or get away and hide to lose your pursuers. Either way, once the key is in hand simply return to the prisoner and set him free to earn your first protégé.