Akey element in the easterlin hypothesis is that the relative size of cohorts can influence the way in which people and societies change over time.

That statement is True.
In this context, Cohort refers to the group of people that shared a certain similar characteristics.
The larger the size of the Cohort, the smaller the chance of the members in that society to be involved in conflict and conducted act of violence towards one another, which would determine the dominant group in that society.

The correct answer is a. when they started making tools that will them in cultivating lands, they could raise crops and that made them attached to one place for a longer period of time. after that, they started domesticating animals and building their own permanent settlements that resemble the ones we have today.