A magnet can be demagnetized by
A. Hammering the magnet
B. Putting it in water
C. Cooling it
D. Putting it in contact with iron

Hint: We are asked how we can demagnetise a magnet. We have several ways to demagnetize a magnet. We have temporary magnets and permanent magnets. Temporary magnets will easily lose their magnetic strength when we take them out of their magnetic fields.

Complete step-by-step solution:
There are many different ways to demagnetize a magnet.
As we know, a magnet has magnetic moments inside it, i.e. the arrangement of molecules in a particular direction.
When we repeatedly hammer on a magnet, it will free the magnetic dipoles inside the magnet from its ordered orientation.
We know that the existence of magnetic moments causes magnetism.
So when we hammer it, the dipoles get disturbed, lose their orientation, and thus magnetic moments no longer exist. Thus the magnet will get demagnetized.
Hence the correct answer is option A.

Additional information:
When a magnet gets demagnetized, the orientation of the magnetic dipoles gets disrupted.
Demagnetization also occurs naturally over time. The speed of the natural process depends on the material, temperature, etc. For some magnets, natural demagnetization is an extremely slow process but for some other magnets, it takes a long time.
If we keep a bunch of magnets together or randomly rub magnets against each other, each will affect the other. Thus it changes the orientation of the magnetic dipoles and lessens the net magnetic field strength.
We can use a strong magnet to demagnetize a weak magnet.

Note: There are some alternative methods to demagnetise a magnet.
Some of them are:
By heating, i.e. heating the magnet past the temperature called Curie point. It has the same effect as when we hammer. So is applying pressure, dropping the magnet to hard surfaces, etc. By applying AC current to a magnet. We know that alternating current rapidly switches direction. So when we apply AC to a magnet, we know the direction of the electromagnetic field changes rapidly, hence the magnetic dipoles try to orient according to the field. Thus they end up randomized and therefore get demagnetized.