I think it is d. Persian empire but I could be wrong :/
The answer is
b.24.
.................
They unified after Oda Nobunaga re-established the centralized government in which there was a strict hierarchy of people who were led by a Daimyo, that is, a feudal lord.
They were technically free. But the Fugitive Slave Laws said that any escaped slave must be returned. So if they were caught they had to go back.
I'm sure it's B. Or D. I would go with B.