Final answer:
During the reign of Mansa Musa, Timbuktu became a key trading city and Islamic center, which helped Islam expand throughout western Africa. So, the correct answer is option d.
Step-by-step explanation:
During the reign of Mansa Musa, the key trading city and Islamic center that helped Islam expand throughout western Africa was Timbuktu. Mansa Musa transformed Timbuktu into a center of international trade and education.
He brought Arab scholars, architects, astronomers, poets, lawyers, mathematicians, and theologians to the city, establishing over one hundred schools and eighteen universities for Islamic studies. Timbuktu's position as a major trade hub along the trans-Saharan trade routes enabled its influence to spread throughout western Africa.