You are correct, the answer A) heeling magnet should be inverted is the correct answer. When crossing the magnetic equator, the Earth's magnetic field changes direction, and to maintain the accuracy of the compass, the heeling magnet should be inverted. The heeling magnet is used to compensate for the effects of the ship's heel or list, and inverting it helps to maintain the compass's accuracy as the magnetic field changes. The other options are incorrect because they do not directly address the change in the Earth's magnetic field when crossing the magnetic equator. Moving the Flinders bar or rotating the quadrantal spheres may be necessary for other compass adjustments, but they are not the primary actions required when crossing the magnetic equator.