The correct option is D Availability of thermostable DNA polymerase
In the PCR, the process of replication of DNA is repeated many times, the segment of DNA can be replicated to approximately 1 billion copies and this is achieved by the use of thermostable DNA polymerase which is isolated mainly from thermostable bacteria like Thermus aquaticus. The availability of this thermostable DNA polymerase popularised the PCR. So the correct option is D.