The correct option is B By rubbing a magnet on it for a few minutes.
We can convert a rectangular piece of iron into a magnet by rubbing it with a piece of a magnet over it for a few minutes. The direction of rubbing should be in the same direction. After that iron piece will also act as a magnet. There are different ways through which this can be done. Single touch method is a method of magnetizing a magnetic substance by rubbing with a magnet. Double touch method is a method of magnetizing a magnetic substance by rubbing with a pair of magnets.