In the process of extraction, gold combines with cyanide in the presence of oxygen and water to make a complex. The formula of the complex is

$[Au(CN)_2]^{-}$

The correct answer is Option (2) → $[Au(CN)_2]^{-}$

Step 1: Recall the process

• Gold is extracted by cyanidation (the MacArthur–Forrest process).
• In the presence of oxygen and water, gold reacts with cyanide:

$4Au + 8CN^- + O_2 + 2H_2O → 4[Au(CN)2]^- + 4OH^-$

• Here, gold forms a linear complex with cyanide.

Step 2: Identify the complex

• The complex formed is: $[Au(CN)2]^-$

This is a monoanionic complex of gold.