I need help with this Question Asap, its for a test!!

The human eye contains a molecule called 11-cis-retinal that changes shape when struck with light of sufficient energy. the change in shape triggers a series of events that results in an electrical signal being sent to the brain that results in vision. the minimum energy required to change the conformation of 11-cis-retinal within the eye is about 164 kj/mol.

Which element is least likely to undergo a chemical reaction

Which of the following is a physical change? a.burning a piece of wood b.sawing a piece of wood in half c.rust forming on an iron fence d.a copper roof changing color from orange to green