Complex formation method is used for the extraction of:
(A) $Zn$
(B) $Ag$
(C) $Hg$
(D) $Cu$

Hint: In complex formation method, an ore is converted into its pure form by the formation of a complex. The transition metals can be extracted through this method. The transition method can form complexes because they are small in size and have d-electrons to form new bonds. They can receive lone pairs of ligands and form complexes because of their small size, d-electrons, and high nuclear size.

Complete Step by Step Answer:
Out of the given options, only silver ($Ag$) can be extracted by a complex formation method. An ore of silver is converted into pure silver with the help of a complex formation method. An ore of silver ($Ag$) is treated with sodium cyanide ($NaCN$) by passing an air current through it and gives silver sodium cyanide ($Na[Ag{{(CN)}_{2}}]$). The silver sodium cyanide is then treated with zinc to form pure silver.

Correct Option: (B) $Ag$.

Additional Information: Metals exist in nature in their impure form, i.e., ore. The pure metal is then extracted from the ores. Different types of ores can be converted into their pure form by different methods of extraction. Metal extraction methods include chemical separation, froth floatation, calcination, roasting, magnetic separation, hydrolytic method, and others.

Note: The extraction of other elements given in the option is done by other methods. The extraction of zinc ($Zn$) and copper ($Cu$) is done by the electrolysis method. The extraction of mercury ($Hg$) is done by the distillation process.