The correct option is
B They are unable to interbreed
Species is group of individuals that can potentially interbreed to produce fertile offspring. Main criteria of putting two individuals under same species is their ability to interbreed to produce fertile offspring, no difference in structure or function of a protein; gibbons and human can not interbreed and are kept under different species. Gibbons and human share common ancestor and are result of divergent evolution.
Therefore, the correct answer is option C.