One-word substitute: That which cannot be passed through:

Correct Answer: A. Impassable

The term “impassable” is used to describe something that cannot be passed through or crossed. It refers to a barrier, obstacle, or route that is blocked or inaccessible, making it impossible or extremely difficult to pass. An impassable terrain, for example, may include rugged mountains, dense forests, or deep water bodies that hinder passage.