Which best describes the term five pillars?
a. the successors to muhammad who spread his teachings
b. the columns that support the entrance to a mosque
c. the caves near mecca where muhammad had a vision
d. the special religious duties a muslim must observe
d?

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.

The correct answer is d). It also inludes giving to charity and a pilgrimage to Mecca.