Griffin performed experiments demonstrating that when live, nonpathogentic, S. pneumoniae (which produce rough-surfaced colonies) are mixed with killed smooth-surfaced S. pneumoniae (which are pathogenic when alive) and are then injected into mice, the mice become ill. Bacteria isolated from these sick mice form smooth colonies characteristic of the pathogenic strain. What happened to the bacteria to make them pathogenic to the mice?

Transformation

Explanation:

The experiment of Griffith on S. pneumoniae leads to the conclusion that the bacteria can take the DNA from the surrounding environment by the process of "Transformation".

He selected the two strains of bacteria, the smooth pathogenic strains (S strains) and the rough non-pathogenic strain (R strains). When he injected the dead S bacteria and live R bacteria, he obtained the live S bacteria from the mice.

He was amazed at how the dead S strain bacteria became alive and concluded that the dead S strain bacteria were transformed into the live S strain bacteria and named the process transformation. The transformation process refers to the uptake of the DNA material from the surrounding environment.

Thus, transformation is the correct answer.