Final answer:
The domestication of the camel led to Arabs populating more of the desert in the first millennium B.C., providing them with strategic mobility for military campaigns and transport over long distances.
Step-by-step explanation:
The direct cause that led to Arabs populating more of the desert in the first millennium B.C. is B. The domestication of the camel. Camels provided a major tactical advantage to Arab armies by allowing them to travel over long distances through deserts with speed, launch sudden attacks, and transport supplies efficiently. This mobility was key to the successful Arab expansion across vast swathes of territory, as it allowed them to outmaneuver and overpower stationary empires like the Byzantine and Persian empires. As the Arab-Muslim community grew and sought out new resources, the domestication of camels enabled them to spread across the Arabian Peninsula into North Africa and beyond.