The correct answer is A) being struck by lightning. The permanent magnetism of a vessel can change in polarity due to the vessel being struck by lightning. Lightning can induce strong electrical currents in the metal parts of the vessel, which can alter the magnetic alignment of the vessel's permanent magnets, causing a change in polarity. The other options are incorrect because: B) being moored for a long time on one heading would not change the permanent magnetism; C) steaming from one magnetic hemisphere to another would not change the permanent magnetism; and D) loading a homogenous magnetic cargo would not change the permanent magnetism, as this would only affect the vessel's induced magnetism, not its permanent magnetism.