Answer:
I would say A. protection from attack and disease.
Step-by-step explanation:
They definitely did not provide fast or easy transportation routes.
they may have provided resources for building homes in some cases, but not in all cases such as the deserts and seas..
and they may have provided fertile farmland for crops in some places, but once again not in all of them such as the deserts and seas, and certain mountains as well.
so that leave protection from attack and disease, which any of them could very well do, because as stated, they were not easy means of transportation. any outside group of travellers or attackers would have to cross a large, rushing river, mountains, miles and miles of desert or plateau land, or an ocean, in order to reach whichever civilization is in question. none of these would be an easy journey on foot, and it'd likely not be worth it, or end up killing off most of the people trying to get there. therefore it would protect them from any newcomers bringing in new diseases their immune systems weren't used to, as well as from most attackers seeing as the attackers would likely end up exhausted, dehydrated, starved, and half dead already, if they made it there at all.... just my opinion though. ...