a. shape-it looks like a triangle.
b.shape-The form confused me a little, but the thickness of the "line"
makes it look like a shape.
c.shape-It is a clear solid rectangle.
d.shape- Because it has random protruding shapes attached to a line
so it is a shape.

I think characters of what makes a line is that they are thin, and they don't connect to make a shape. Contour drawings are good example, as they make a shape, but it is still a line as it is not finnished off to the end.
Shapes are defined by it's solidity. Even lines that create the shape are shape when they are ended up in one point. I think many times, people consider shape if it is recognizable; if the shape resembles triangle, then they would immediately judge it as a shape.