Which of the following is an example of a geometrical line?

A line can be defined as a straight, one-dimensional figure that extends infinitely in both directions. 

When two walls meet, they form a straight line where the two surfaces intersect. This line is the boundary between the two walls and extends infinitely in both directions, theoretically. Therefore, we can say that the meeting place of two walls is an example of a geometrical line.

In terms of geometric shape, the blackboard and a sheet of paper are rectangles, while the tip of a sharp pencil is a point.