Current flows through uniform, square frames and their connecting wires as shown in the figure. In which case is the net magnetic field at the centre of the frame not zero?

Step-by-step solution

The square frame itself always produces zero net field at the centre (symmetric current paths cancel). Cases A and B: External wires are directed radially (along diagonals) toward/away from the centre their line of action passes through the centre, so they produce zero field there. Case D: Incoming and outgoing external wires are on opposite sides and symmetric their fields cancel at the centre. Case C: Both external wires are collinear with the top edge and carry current in the same direction (left to right). By t