Which one of the following is not a method of concentration of ore

Smelting is the process in which the concentrated ore is heated in the presence of a reducing agent like Carbon to extract the desired metal. Hence, it is not a process for the enrichment of ore. 

Whereas, other given process such as Froth floatation, electromagnetic separation, and gravity separation are few of the processes used for ore concentration.

Therefore, Option D is the correct answer.