In which region should a MOSFET be operated as a switch?
When your article says this (wrongly): -


  MOSFET in saturation region is preferred to make it work as a switch.


It's because it's written by someone who thinks that the name of the equivalent section of the BJT's characteristic is 100% transferable to MOSFETs.

To clear this up: -


When a MOSFET is operated as an on-switch it works in the triode or ohmic region
When a MOSFET is operated as an off-switch it works in the cut-off region
When a MOSFET is operated as a controlled current device it works in the saturation region
"Saturation" refers to the channel being saturated





When a BJT is operated as a switch it works in the saturation region and cut-off regions
"Saturation" in the case of a BJT refers to the saturation of the base in that both PN or NP junctions are (somewhat) conducting





  Should I operate the MOSFET to "Turn ON" in a (Linear/Ohmic/Triode) or
  Saturation region?


Answer: the linear/ohmic/triode region