The surest test for a magnetism is
repulsion
Explanation for correct option:
option a)
1. A magnet attracts things made up of iron, cobalt, and nickel or their alloys.
2. Every magnet has two poles. The north pole and a south pole.
3. Similar poles repel each other and opposite poles attract each other.
4. Thus a magnet attracts the objects made up of iron, cobalt, and nickel or their alloys as well as it also attracts the opposite pole of another magnet when brought near to it.
5. A magnet repels only the similar pole of another magnet when brought close to it
6. Repulsion is the surest test of a magnet.
Explanation for incorrect option:
option b)
Thus, option a) repulsion is the correct answer.