The correct answer is option 3: silver sulphide.

Silver artefacts tend to tarnish or become dull and discolored when exposed to air. This tarnishing occurs due to the formation of a layer of silver sulphide on the surface of the silver.

When silver comes into contact with air, it reacts with small amounts of hydrogen sulphide (H2S) present in the atmosphere. This reaction leads to the formation of silver sulphide (Ag2S), which appears as a black or dark grey layer on the surface of the silver object.

It is important to note that the formation of silver chloride (option 1), silver oxide (option 2), and silver sulphate (option 4) are less common in the tarnishing of silver compared to silver sulphide. While silver chloride and silver oxide can also contribute to the tarnishing of silver, they are generally less significant factors.

Overall, the answer 3 - silver sulphide - best explains the tarnishing process of silver when exposed to air.