Why is mercury obtained from cinnabar ore through two heating steps instead of direct reduction?

The correct option is: Mercury sulphide must first change into oxide.

Mercury is a metal of low reactivity positioned near the bottom of the activity series. Its primary ore is cinnabar, which is chemically mercury(II) sulfide (HgS). In metallurgy, it is generally much easier to reduce a metal oxide to its metallic form than it is to reduce a metal sulfide directly. Therefore, the extraction process involves two distinct heating steps:

Step 1: Conversion of sulfide to oxide (Roasting)
When cinnabar (HgS) is heated in the presence of excess air (oxygen), it undergoes roasting. During this step, mercury sulfide is oxidized to form mercury(II) oxide (HgO), while sulfur dioxide gas is released as a byproduct:
$$2\mathrm{HgS}\left(s\right)+3O_2\left(g\right)\to 2\mathrm{HgO}\left(s\right)+2{\mathrm{SO}}_2\left(g\right)$$

Step 2: Thermal reduction of the oxide
Mercury(II) oxide is thermally unstable. Upon further heating (without requiring any additional reducing agent like carbon), it decomposes to yield liquid mercury metal and oxygen gas:
$$2\mathrm{HgO}\left(s\right)\to 2\mathrm{Hg}\left(l\right)+O_2\left(g\right)$$

Because of this chemical pathway, direct reduction of mercury sulfide to mercury metal is not practically viable. Instead, the sulfide ore must first be converted into the metal oxide before the metal can be isolated.