The above process is:
\[HgS\xrightarrow{{{O_2}}}HgS.HgO\]$\xrightarrow{\Delta }Hg$
 A. Auto oxidation
B. Self reduction
C. Thermal reduction
D. None of the above

Hint:Mercury sulfide is a chemical element of mercury and sulfur. It is represented by the formula \[{{\mathbf{H}}_2}S\]. It is also called cinnabar and it is virtually insoluble in water. The crystals of red are optically active. The is caused by the helices in the structure. Black powder is precipitated when \[{{\mathbf{H}}_2}S\] is bubbled through solutions of \[{\mathbf{Hg}}\left( {{\mathbf{II}}} \right){\text{ }}{\mathbf{salts}}\]. \[{\mathbf{\beta }} - {\mathbf{HgS}}\]is unreactive to all but concentrated acids.

Complete step-by-step answer:It is dimorphic with its two crystalline forms:
1.Red cinnabar- mostly found mercury in nature
2.Black, metacinnabar-less common in nature and adopts zinc blende crystal structure
The production of mercury can be done from cinnabar by roasting it in air and condensation of vapour. This releases sulfur dioxide. Then the oxide formed decomposes to pure metal. This means oxygen gas will be released. Hence, when cinnabar is heated, pure mercury is obtained. Mercury is a naturally obtained metallic element and is often found in air, water and soil. Mercury exposure may lead to serious health issues such as health problems and is a threat to child development. The toxic effects of mercury include damage to the brain, kidney, lungs, nervous, digestive and immune systems, skin, and eyes.
Cinnabar when heated with oxygen gets reduced to mercury without need of any additional reducing agent. The process is known as self-reduction. Process here is $B$self reduction

Note:Mechanical pressure gauges and electronic strain gauge sensors have replaced mercury sphygmomanometers. It is often used in research applications and for dental restorations. Electricity when passed through mercury vapour in fluorescent lamps produces short-wave ultraviolet light. Mercury can be used in manometers, thermometers, fluorescent lamps and other devices.