Transformation in diplococcus by heat killed extract was discovered by
D)Mc Leod

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Hint: The look for genetic material started during the mid-nineteenth century. The principle of inheritance was revealed by Mendel. Based upon his investigation Mendel came into the conclusion that some factors are transferred from one generation to another.

Complete answer:
 Mendel’s Law of Inheritance was the foundation for the researchers on genetic material. Maintenance of his conclusions in mind the scientists who came after him paying attention to chromosomes in search of genetic material. Even if the chromosomal components were identified the material which are responsible for inheritance remained unreturned. It took a very long time for the acceptance of DNA as the genetic transformation.
Griffith Experiment and the Transforming Principle
Griffith experiment was a stepping stone for the finding of genetic material. Frederick Griffith experiments were played on Streptococcus pneumoniae.

Through the experiment Griffith cultured Streptococcus pneumoniae bacteria which exhibited two patterns of growth. One culture plate composed of smooth shiny colonies at the same time as others consisted of rough colonies . The dissimilarity was due to the presence of mucous coat in S strain bacteria while the R strain bacteria lacked them.
The Experiment is as follows
Griffith injected both S and R strains into the mice. The one which is infected with the S strain developed pneumonia and died even as that infected with the R strain stayed alive.
In the next stage Griffith heat killed the S strain bacteria and injected into mice but in this stage the mice stayed alive. After that he mixed the heat-killed S and live R strains. Then he injected this mixture into mice and they died. In adding up he found living S strain bacteria in dead mice.

Based on the observation Griffith came into the conclusion that R strain bacteria had been transformed by S strain bacteria. The R strain bacteria inherited some transforming principle from the heat-killed S strain bacteria which created them virulent. And he understood this transforming principle as genetic material.

Hence, the correct answer is option (B)

Note: Griffith experiment was a major turning point towards the discovery of hereditary material. Though it failed to explain the biochemistry of genetic material. Therefore a group of scientists, Oswald Avery, Colin MacLeod and Maclyn McCarty sustained the Griffith experiment in search of biochemical nature of the hereditary material. Their innovation revised the concept of protein as genetic material to DNA as genetic material.