In the Ground Zeroes mission, rescued the prisoner to be executed and extracted him via chopper

Location: The prisoner does not appear until after your mission has progressed past saving Chico. Down the hill from where you start the mission, make your way to the far-left side beyond all the tents. Be on the lookout for a pile of crates, next to them the prisoner in a yellow jumpsuit will be lying down.

Pick up the prisoner and carry him back to where you started the Ground Zeroes mission. By the way, its a good idea to call in the chopper via the (in-game) iDroid menu before you reach the starting area (for example: once you get out of the compound gates again), so that the chopper will arrive shortly after you get there.

This video guide shows you the shortest route to take to the prisoner and back again to the evac point.