Final answer:
The tarnishing of a penny occurs due to oxidation, where the copper reacts with oxygen to form copper oxide.the correct option is A.
Step-by-step explanation:
The tarnishing of a penny, or any copper-containing coin, is the result of oxidation. When the penny is exposed to air, the copper in the penny reacts with oxygen to form copper oxide, which is a darker substance that makes the penny appear tarnished. This is similar to how silver tarnishes when exposed to sulfur, forming silver sulfide as indicated by the reaction given: 2 Ag(s) + H₂S(g) → Ag₂S(s) + H₂ (g).
In contrast to other processes like evaporation, condensation, and sublimation, which involve the physical state changes of water as influenced by heat from the sun, oxidation is a chemical process involving the transfer of electrons between elements. Through oxidation, metals like silver and copper undergo a chemical change that alters their appearance, which is not caused by sun the same way the water cycle is affected.