The correct answer is B) adoption of a major religion.
The option that was a common factor to most of the African kingdoms by the 12th century was the adoption of a major religion.
In the 12th century, the Kingdom of Mapungubwe, in Southern Africa, started to rise. It represented the foundation of the Kingdom of Zimbabwe. Another important kingdom was the Akan Empire of Bonoman, that inhabited parts of the Ivory Coast and Ghana. This was the origin of the Kingdom of Ashanti. By this time in history, a common factor to most of the African kingdoms was the adoption of a major religion.