What is the impact of the bantu migration?

Causes of Bantu Migration: 
Farming Technique
Developing Agriculture
Area were becoming desert like
Using more land for farming which, caused less living space for people.

Effects of Bantu Migration:
Defending their life and land
Territorial wars began
Resource Depletion
Brought new cultures and different ideas.
Populated the world with different blends of people.

Positive Impact of the Bantu Migration:
Introduction of Iron working, and the use of iron tools throughout much of Africa, and when iron-smelting started , there was an increased in food production.

Introduction of new crops e.g. Yams, bananas, the Bantu increased their knowledge of food and crop cultivation. Earlier on in Africa, the inhabitants were hunters and gatherers, but with iron smelting, food production began.

They introduced centralized administration. They introduced a centralized system of Government where the King acted as the overall ruler, under whom were the other chiefs.

Interaction with other tribes: Bantu language became widespread in East Africa.

Introduced a system of building permanent homes: The opening of the new land to raise families, and setup villages , and clans.

Defense: The Knowledge of iron smelting led to the making of bows, arrows, and spears for defense, and protection. 

Unified Regions: Migrations led to rise of large states and bigger tribes in East Africa, e.g., the Baganda, Kikuyu, Nyamwezi...

Negative Impacts on the Bantu Migration:
Depopulation: This was caused by the frequent attacks made by the Bantu against the people of East Africa for land through wars.

Loss of Culture: Cultural Absorption resulted in the loss of culture., Also, Bantu intermarrying with the Non- Bantu peoples was a factor.

Outcome:
The Bantu Migration being the largest migration in History, influenced two-thirds of African language. By the end of the First Millennium divisions had formed. There was a difference between the rich and poor, as well as with men and women. The Bantu were the first to develop language, and provide historians, with an idea about Africa's Civilizations. The Bantu were also, the First Farmers, and Cattle Keepers.

Hope this helps!

The answer should be d.