Below, there is a sequence of images showing how the original diagram gets transformed into the diagram in section (c), by applying these simple principles:
The key point to remember here is that any two points that are directly connected by a wire are electrically the same point.
So first ignore any wires that just ‘stick out’ from the points A, B, C and D. They don’t add any information.
Then notice that B and C are connected by a wire and so in electrical terms B and C are the same point. So erase the point C and redraw C at the same position as B.
Then imagine that you can pick up the point D and move it to the left of the point A without breaking any of the connections.