Achange in color, such as rusting of metal, is a sign
a.) a chemcal change is taking place.
b.) a physical change has just taken place.
c.) oxygen is present
d.) organic chemicals are present

A change in colour such as rusting of metal is a sign that a chemical change is taking place on the surface of the metal.

Option A

Explanation:

Rusting is the product of chemical reaction occurring on the surface of any metals when it comes in contact with elements like oxygen, water or other oxidising agents.

These oxidising agents remove the electrons from the surface on which they are reacting and form a layer of precipitate as a product of the oxidation reaction, thus changing the colour of the substance.

Any reaction where a new substance formed is termed as chemical change or chemical reaction occurring on the metal. So in this case also, the change in colour of the metal is a sign of chemical change.