Answer: A. They needed to finance travel to India.
Explanation: The Portuguese were exploring and establishing trade routes to India, and they needed funds to support their expeditions. By demanding tax payments from the East African trade cities, they were able to generate revenue that could be used to finance their voyages to India. These tax payments helped cover the costs of maintaining fleets, funding expeditions, and establishing trading posts along the way.
This financial support was crucial for the Portuguese as they sought to compete with other European powers in the lucrative Indian Ocean trade. By demanding taxes from the trade cities in East Africa, the Portuguese were able to secure the necessary resources to continue their exploration and establish their presence in the region.