Which best describes the term, Five Pillars?

the successors to Muhammad who spread his teachings

the columns that support the entrance to a mosque

the caves near Mecca where Muhammad had a vision

the special religious duties a Muslim must observe

Answer: D. the special religious duties of a muslim must observe.

Explanation:

One of the Five Pillars of Islam is the prayer - so it's an activity, not a place or an object or people. Because of this already we can reject the first three options: the first one describes a person, the second an object and the third a place.