Auto-reduction process is used for the extraction of:
A) $\text{ Cu }$
B) $\text{ Hg }$
C) $\text{ Pb }$
D) $\text{ Al }$

Hint: Auto reduction is an industrial process of extraction. Auto reduction process in which the sulphide ores of less electropositive metal are heated in presence of air and converts the sulphide ore into sulphate or oxide ore.

Complete Solution :
- The auto reduction or self-reduction process is a metal extraction method. In this process a sulphide ore of less electropositive metal like copper $\text{ Cu }$ , lead $\text{ Pb }$ , mercury $\text{ Hg }$ , etc. In this process, the metal sulphide ore is heated in presence of air. It is done by the conversion of ore into sulphate or oxide. These further react with the remaining ore of the sulphate in the absence of the air to give the less electropositive metal and the sulphur dioxide.
- In simpler words, the auto reduction process is used for the preparation of less electropositive metal like copper, mercury, lead, etc.

- Some of the examples of auto reduction are as follows:
The copper ore is roasted in presence of air to give oxide. After a time, the air supply and the temperature is raised such that the left sulphide ore is converted to form metal. The reaction is as follows:
$\begin{align}
  & \text{ 2CuS + 3}{{\text{O}}_{\text{2 }}}\to \text{ 2CuO + 2S}{{\text{O}}_{\text{2 }}}(\text{limited supply of air)} \\
 & \text{ CuS + 2CuO }\to \text{ 6Cu + S}{{\text{O}}_{\text{2}}}(\text{in excess of air)} \\
\end{align}$

- Similarly, the lead can be extracted through the auto reduction method. The extraction of lead is as shown below:
$\begin{align}
  & \text{ 2PbS + 3}{{\text{O}}_{\text{2 }}}\to \text{ 2PbO + 2S}{{\text{O}}_{\text{2 }}}(\text{limited supply of air)} \\
 & \text{ PbS + 2PbO }\to \text{ 3Pb + S}{{\text{O}}_{\text{2}}}(\text{in excess of air)} \\
\end{align}$

Similarly, mercury can be extracted through the auto reduction method. The extraction of lead is as shown below:
$\begin{align}
  & \text{ 2HgS + 3}{{\text{O}}_{\text{2 }}}\to \text{ 2HgO + 2S}{{\text{O}}_{\text{2 }}}(\text{limited supply of air)} \\
 & \text{ HgS + 2HgO }\to \text{ 3Hg + S}{{\text{O}}_{\text{2}}}(\text{in excess of air)} \\
\end{align}$

Thus, copper, lead, and mercury is extracted through auto reduction.
So, the correct answer is “Option A, B and C”.

Note: Note that, the auto-reduction method is used for the extraction of less electropositive metals only. The process does not require any external reducing agent that is why the process is termed as “Auto-reduction”. Here the metal sulphide and metal oxide reduce the metal to its elemental form.