Microprocessors as switching devices are for which generation computers

The generations of computer are classified into 5 generations.

First generation of computers: Vaccum tube based computers.

Second generation of computers: Transistor based computers. 
Third generation of computers: Integrated Circuit based computers.

Fourth generation of computers: VLSI microprocessor based computers
Fifth generation of computers: ULSI microprocessor based computers. 

Hence, correct option is D i.e., Fourth Generation.