the most effective way of demagnetizing a magnet is Electrical method. option c is not correct since battery instead of a solenoid . option A  is another method but the placing of the magnet is wrong it should of have been in the east-west direction not north-south direction. option C is not a method. so that leaves us with OPTION d which is also a method of demagnetizing a method, so when a magnets are strongly heated they lose their magnetism. therefore OPTION D is correct.