Answer:
The correct answer would be the harmless bacteria had been transformed.
Griffith used two different strains of the bacteria <em>Streptococcus pneumoniae - </em>type-III-S or smooth strain and type II-R or rough strain.
Smooth strain had protective covering around itself (protect itself from hosts's immune system) and was able to kill the mice.
Rough strain did not have any protective covering around itself and thus could be easily removed by the immune system Hence, it was not able to kill the mice.
In addition, heat killed smooth strain was also not able to kill the mice. However, when remains of it was added with rough strain then the blend was able to kill the mice.
Lastly, he was able to isolate living bacteria of both the strains.
He concluded that non-lethal type II-R strain was transformed into lethal type II-S strain by "transforming principle" (which we know today as DNA) that was supposed to be the part of dead III-S strain bacteria.
