Ans: (D) Alfred Hershey and Martha Chase
They grew some virus that contained radioactive phosporus and some others on medium containing radioactive sulphurs. Viruses grown in the presence of radioactive phosporus contained radioactive DNA but not radioactive proteins because DNA contains phosporous and protein does not. Similarly, viruses grown on radioactive sulphur contained radioactive protein and not radio active DNA because DNA does not contain sulphur. Radioactive phager were allowed to attach E.Coli. Then as infection proceeded, the viral coats were removed from the bacterium by agitating in the blender. Viral particles were separated from bacteria by centrifugation. Bacteria which was infected with viruses that had radioactive DNA were radioactive indicating that DNA was the material that passed from viruses to bacteria.