The correct answer is (d) Mice -> Heat killed S strain + live R strain -> Alive
The explanation: When the heat killed S-strain of the bacterium is given along with the R strain to the mice, a process of transformation occurs. The heat killed S strain (avirulent) was able to transform the R strain (avirulent) to become a virulent strain. During this process of transformation, the heat killed S strained bacterium, transforms the R strain to produce the mucous(polysaccharide) and become virulent.