If the key in the arrangement, in the given figure is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are:
Straight lines parallel to each other
When the key is taken out, there is no current flowing through the straight conductor. Hence, there is no magnetic field due to it in the plane ABCD. Only Earth's magnetic field, which is uniform, is present in the plane ABCD and the field lines due to it are straight lines parallel to each other.