The aboriginal people ended up being displaced by settlers who justified themselves with the fact that the displaced people didn't have a formal system to do what?
spend time own land own home
own land
being displaced typically suggests losing a place of resident, which can be a home or land, so this suggests both b and c are possible.  however, being displaced by "settlers", which usually is associated with finding new land to acquire and build a life there.  therefore "own land" is the most likely answer.
being displaced by settlers