letter A is a shape because it has three lines that connect and a black fill.
letter B is a line because it has a clear starting point and a clear ending point.
letter C is a shape because there are four lines that connect at the corners and it is filled with black.
letter D is a shape because there is no clear starting point or ending point.

a line should have a clear starting point or at least one direction.
a shape is an aria of space that is occupied by a fill of color or shade.