The correct option is Corrosion.
Explanation
Corrosion is a chemical process in which metals are gradually eaten away or deteriorated by the action of air, moisture, or chemicals (such as acids) in their environment. It is primarily an oxidation process where the metal converts into a more stable form, such as an oxide, hydroxide, carbonate, or sulfide.
Analysis of the Process:
- Corrosion (Correct): The specific changes mentioned in the question are classic examples of metallic corrosion:
- Copper: When exposed to moist air containing carbon dioxide, copper slowly loses its shiny brown surface and gains a green coating. This green substance is basic copper carbonate ($CuCO_3 \cdot Cu(OH)_2$).
- Silver: Silver articles turn black when exposed to air because they react with sulfur compounds (like hydrogen sulfide) present in the atmosphere to form a coating of silver sulfide ($Ag_2S$).
- Rancidity (Incorrect): This process refers to the oxidation of fats and oils in food, leading to an unpleasant smell and taste. It is not related to metals.
- Calcination (Incorrect): This is a metallurgical process where an ore is heated strongly in the absence of air (or limited supply) to remove volatile impurities or convert carbonates into oxides.
- Distillation (Incorrect): This is a physical method of separating mixtures based on differences in the boiling points of the components.
Key Takeaway:
Corrosion is the oxidative degradation of metals due to environmental factors. Distinctive indicators include the green patina on copper (carbonate formation) and the black tarnish on silver (sulfide formation).