The correct option is B Both A and R are true but R is not the correct explanation of A
Lion and tiger belong to the genus Panthera. They belong to the same genus because of various similarities in morphological and other characters. They, however, cannot interbreed successfully to produce a fertile offspring, which puts them under different species, leo and tigris respectively. Thus, the fact that they cannot interbreed is a reason for the different species they belong to, and not the reason for sharing the same genus.