One of the commonly used electrode is calomel electrode. Under which of the following categories calomel electrode comes ?

The correct answer is Metal – Insoluble Salt – Anion electrodes.

Let's understand the construction and classification of the calomel electrode step-by-step:
1. Definition and Construction:
A calomel electrode is a reference electrode that consists of mercury (a metal), a paste of mercurous chloride (${\mathrm{Hg}}_2{\mathrm{Cl}}_2$, which is a sparingly soluble/insoluble salt), and a solution of potassium chloride (KCl, which provides the chloride anion, ${\mathrm{Cl}}^{-}$).
2. Electrode Representation:
The electrode is represented as:
$\mathrm{Pt}\left|\mathrm{Hg}\right(l\left)\right|{\mathrm{Hg}}_2{\mathrm{Cl}}_2\left(s\right)\left|{\mathrm{Cl}}^{-}\right(\mathrm{aq})$
This representation clearly shows that it consists of a metal (Hg) in contact with its insoluble salt (${\mathrm{Hg}}_2{\mathrm{Cl}}_2$) and a common anion (${\mathrm{Cl}}^{-}$).
3. Classification:
Because the potential of this electrode is determined by the concentration (or activity) of the anion (${\mathrm{Cl}}^{-}$) in equilibrium with the insoluble salt, it falls under the category of Metal – Insoluble Salt – Anion electrodes.