The discovery of DNA as the genetic material is a significant milestone in the field of genetics. The efforts of many scientists led to the conclusive proof that DNA is the hereditary material that passes from one generation to another.

The studies on bacterial viruses, also known as bacteriophages, played a crucial role in establishing DNA as the genetic material. The experiments conducted by Alfred Hershey and Martha Chase in 1952 provided the unequivocal evidence that DNA is the genetic material.

Explanation:
• Bacteriophages are viruses that infect bacteria and reproduce inside them.
• The Hershey-Chase experiment demonstrated that bacteriophages inject their genetic material into bacteria, causing them to produce new virus particles.
• The experiment involved two sets of bacteriophages labeled with different radioactive isotopes - one set with radioactive sulfur (35S) and the other with radioactive phosphorus (32P).
• The radioactive sulfur labeled the protein coat of the bacteriophage, while the radioactive phosphorus labeled the DNA inside the bacteriophage.
• The bacteriophages were allowed to infect bacteria, and the infected bacteria were then separated from the virus particles.
• The researchers found that the radioactive phosphorus, which labeled the DNA, was present in the infected bacteria, while the radioactive sulfur, which labeled the protein coat, was not.
• This result provided conclusive evidence that the genetic material of the bacteriophage was DNA, not protein.
• The Hershey-Chase experiment thus established DNA as the genetic material, providing a foundation for further research in genetics.

Therefore, the studies on bacterial viruses, specifically the Hershey-Chase experiment, provided the unequivocal proof that DNA is the genetic material. Thus, option D - bacterial virus is the correct answer.

D