Mapped boundaries that often follow straight lines are referred to as a. territories b. visible boundaries c. natural boundaries d. imaginary boundaries

Mapped boundaries that often follow straight lines are referred to as imaginary boundaries. Correct D Ambrose Bierce defined imaginary boundaries as following : In political geography, an imaginary line between two nations, separating the imaginary rights of one from the imaginary rights of the other.

D. Mapped boundaries that often follow straight lines are referred to as imaginary boundaries.

The straight line are latitude and longitude which are imaginary lines draw all around the globe. The longitude (vertical lines) which runs along north south direction and latitude (horizontal lines) which runs along east west direction.  

The main purpose of these lines (geometric boundaries) is to denote the exact position of a location in the earth. This border is not used politically, only used for scientific purpose.