. To convert an iron bar into a magnet we need
(a) to hammer the iron piece a large no. of times.
(b) red colour to denote north pole.
(c) a magnet.
(d) to heat it.
(c) Explanation: Striking an iron bar along a magnet in the same direction and repeating the process for more than 30-40 times, the iron bar gets converted into a magnet.