Final answer:
The most common factor used to draw new boundaries for African countries was arbitrary lines.
Step-by-step explanation:
The most common factor used to draw new boundaries for African countries was D) Arbitrary lines. During the 1884 Berlin Conference, European powers divided the African continent and created boundary lines between their colonies. These lines were often drawn without considering the cultural heritage, geographic features, or colonial history of the region, leading to the division of communities and conflicts among different ethnic groups.