Step 1: Understanding the Concept:
Tarnishing is a form of corrosion that occurs on the surface of certain metals, such as silver. It results in a dull, often black, layer. The question asks for the chemical identity of this layer on silver.
Step 2: Detailed Explanation:
1. Silver (Ag) is a relatively unreactive metal, but it does react with certain substances present in the atmosphere over time.
2. The most common cause of the black tarnish on silver is its reaction with sulfur-containing compounds present in the air.
3. The primary reactant is hydrogen sulfide (H$_2$S), which has the smell of rotten eggs and is found in small amounts in the atmosphere, especially from pollution.
4. The chemical reaction that occurs is:
\[ 4\text{Ag(s)} + 2\text{H}_2\text{S(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{Ag}_2\text{S(s)} + 2\text{H}_2\text{O(l)} \]
5. The product, silver sulfide (Ag$_2$S), is a black solid that forms a thin layer on the surface of the silver object, causing it to look dull and tarnished.
6. Silver does not readily react with sulfates, nitrates, or chlorides under normal atmospheric conditions to form a tarnish layer.
Step 3: Final Answer:
The tarnishing of silver is due to the formation of a silver sulphide (Ag$_2$S) layer. Therefore, option (C) is correct.