Answer: Hydrogen reacts with oxygen to form water
Explanation: Chemical changes are those which changes the identity of the chemical nature of any substance.
Thus out of the given options , the correct one is the formation of the water molecule when hydrogen reacts with oxygen as it is changing the identity of hydrogen and oxygen atoms to form a molecule of water.
The rest of the three options belongs to the category of the physical changes in which the chemical nature of any substance has not been changed but only the properties which an be easily observed can be changed.