In his work with pneumonia-causing bacteria and mice, Griffith found that
a. The protein coat from pathogenic cells was able to transform nonpathogenic cells.
b. Heat-killed pathogenic cells caused pneumonia.
c. Some substance from pathogenic cells was transferred to nonpathogenic cells, making them pathogenic.
d. The polysaccharide coat of bacteria caused pneumonia.

Griffith's experiment demonstrated the phenomenon of transformation, where a substance from heat-killed pathogenic bacteria could genetically alter nonpathogenic bacteria, making them pathogenic. This experiment was pivotal in suggesting that DNA might be the molecule responsible for heredity, as the transformation indicated a transfer of genetic information.

Transformation is a process where genetic material from one organism is taken up by another, leading to a change in its genotype and phenotype. In Griffith's experiment, nonpathogenic bacteria acquired the ability to cause disease after being exposed to material from dead pathogenic bacteria, indicating the transfer of genetic information.

Pathogenic bacteria are capable of causing disease, while nonpathogenic bacteria do not. Griffith's experiment involved the interaction between these two types, showing that nonpathogenic bacteria could become pathogenic through the uptake of genetic material from dead pathogenic bacteria, highlighting the role of genetic material in determining bacterial virulence.