The correct answers is the following.
The effect that plentiful harvest had on populations in ancient civilizations of the mainlands river valley was the following.
Plentiful harvest allowed abundant food and resources for civilizations in these regions. This increased the number of people and the size of the cities. Rivers helped people to establish and develop agriculture. Plentiful harvest created abundance. We can see this in ancient civilizations like Summer, Mesopotamia. Major civilizations were built between the Rivers Tigris and Euphrates. Another example in Egypt and all the benfits that received from the Nyle River. They could grow and formed a great Empire.