The magnetic properties of a magnet are lost by heating it beyond the Curie temperature (At Curie temperature certain magnetic materials undergo a sharp change in their magnetic properties.). Hammering it repeatedly and  keeping it idle for a long time will also result in the loss of magnetic properties. Immersing a magnet in water doesn't lead to loss in magnetic properties.

A magnetic compass is a tool used for navigation. A magnetic compass aligns in North-South direction. This is because Earth behaves like a huge magnet and the poles of the Earth behave like poles of a huge bar magnet. As such, a freely suspending magnet gets attracted to the opposite poles and rotates in the direction of attraction. This direction is the North-South direction.

Magnetic materials like cobalt, nickel and iron are always attracted by magnets. Magnets cannot repel them, as magnets can only repel other magnets because they have poles, and like poles repel each other.

Every magnet has two poles i.e North pole and South pole. Unlike poles of magnets attract and like poles of magnets repel each other. So, when the South pole of a bar magnet is brought near the North pole of a ring magnet, they will attract each other.

Materials which gets attracted to a magnet are called magnetic substances. Materials which don't get attracted to a magnet are called non-magnetic substances. Glass, wood and chinaware are not attracted to magnets, so, they are non - magnetic materials. Nickel is attracted to a magnet, hence, it is a magnetic material.  

Materials which gets attracted by a magnet are called magnetic materials. In the given options only iron is a magnetic material. Hence, an iron nail gets attracted to a magnet and the rest don't.

Lodestone is a naturally occurring magnet. In ancient times, it was used as a compass to determine directions. A lodestone, when suspended aligns in North-South direction. 

An electromagnet is a type of magnet in which the magnetic field is generated with the help of an electric current. Magnetic materials can behave like electromagnets if a current is passed through a coil around them. It is a temporary magnet, i.e. it loses its magnetic properties as soon as the current flow is stopped. Electric bell has an electromagnet in it which attracts the striker.

Magnetite, a naturally occurring mineral. It is an ore of iron, which has magnetic properties. Hence, it is the only natural magnet. The rest of them are magnetic materials but not magnets.