Which of the following is a metal?

The correct option is Mercury.

To identify the metal among the given options, let's examine the classification of each element in the periodic table:

1. Carbon: Carbon is a non-metal. It belongs to group 14 of the periodic table and is known for forming covalent bonds. In its elemental form, it exists as solids like graphite or diamond, which are typical non-metallic structures.
2. Mercury: Mercury is a transition metal (with the chemical symbol Hg). Although it is unique for being a liquid at standard room temperature and pressure, it possesses all key metallic properties such as good electrical and thermal conductivity, high density, and luster.
3. Sulphur: Sulphur is a non-metal. It belongs to the oxygen family (group 16) and is a brittle, yellow solid at room temperature that is a poor conductor of electricity.
4. Iodine: Iodine is a non-metal belonging to the halogen group (group 17). It is a shiny, dark-gray or purple solid at room temperature.

Therefore, Mercury is the only metal among the options provided.