Which type of the MOSFETs is exclusively used by MOS digital ICs?

The correct answer is option 1):(Enhancement MOSFET)

Concept:

• A MOSFET (metal–oxide–semiconductor FET) is one kind of field-effect transistor with an insulated gate mainly used for amplifying or switching signals. 
• This is used in Digital ICs.MOS ICs use enhancement MOSFETs exclusively.
• The features of these MOSFETs are low power dissipation, simple manufacturing, and small geometry. So these features will make them used within integrated circuits
• There is no pathway in between the drain (D) and source (S) of this MOSFET when no voltage is applied in between the gate & source terminals.
• So, applying a voltage at gate-to-source will enhance the channel, making it capable of conducting current. This property is the main reason to call this device an enhancement-mode MOSFET.