At the supermarket checkout line, you place a bar between your groceries and the groceries of a person behind you, which is an example of
a. central markers
b. land markers
c. space markers
d. boundary markers

Letter d is correct. Boundary markers.

Explanation:

Boundary markers are physical markers to identify boundaries, the alteration or the beginning of a boundary. They emerged as a form of social hierarchy to declare boundaries over a land space.

There are several types of boundary markers, they can be described as pillars, obelisks, corners and contour trees.  The purpose of these boundary markers is according to Josiah Ober "a way to impose human, cultural and social meanings on a once undifferentiated natural environment."