Iron can rust when it is exposed to oxygen. what method could be used to prevent iron from rusting?

You have a few choices:

-- Take it out of Earth's atmosphere.

-- Put it in a tank where you pumped all the air out of it.
   You can put whatever you want back in the tank, just
   as long as there's no oxygen in there.

-- Besides oxygen, rusting also needs water.  The humidity
   (water vapor) in the air is enough to do it.  So if you can
    keep the air totally, totally dry around the iron, then the
    oxygen alone won't make it rust.

-- All these choices do the same thing:  Make sure that
   oxygen and water vapor can't reach the iron.  The easy
   way to do that is to paint or spray something onto the
   surface of the iron that keeps the air away from it.