The point over which a level is centered is called a
a. station mark
b. transit
c. plumb line
d. point of reference

  D.  Point of reference

Explanation:

A point of reference used as a starting point for determining locations of objects at a construction site or in an area being surveyed will often be located on the above-ground portion of a station mark. The transit used for making the survey measurements will be leveled, and centered over the point of reference using a plumb line.