During fixing the metal rim on a wooden wheel of a cart, which of the following is used to contract the metal rim? 

For a metal rim to be fixed to a wooden cart wheel to function well, the wooden cart wheel must be linked to the metal rim in some way. And it must not come apart, no matter how hard it is. When heated, a hoop constructed from a flat iron or steel band will similarly expand.
After being heated, the hoop significantly stretches and may be placed around the wheel. Then it quickly cooled, shrank back to its regular, unused size, and gripped the wheel so firmly that it could not be removed. They are heated because the expansion that occurs when the iron ring is heated is what is known as expansion. Its size will likewise expand as it expands, so after it is fitted, it should fit in the wooden wheel. The iron rim needs to cool down so that it will fit very tightly.

So, option A is the correct option.