The correct answer is:
D. It was the site of the Kaaba, where pilgrims congregated.
Explanation:
The city of Mecca is the holiest city in the religion of Islam because it is where Muhammad was born and because The Kaaba (the most important mosque which was built in a cuboid form by Abraham and his son Ishmael) is located in this city. Mecca is considered the place where angels visit and mercy falls.
In the religion of Islam, there are five pillars (faith, five daily prayers, charity, the fast of Ramadan and pilgrimage). The Hajj is the annual pilgrimage where Muslims go to the Mecca mainly to visit The Kaaba during the last month of the Muslim lunar year, and lasts for five days. Muslims face the direction of The Kaaba when they pray not because they worship The Kaaba, but because it is the place of worship of the one God, Allah.