Cardio-respiratory fitness refers to the ability of the circulatory and respiratory systems to supply oxygen to skeletal muscles during sustained physical activity.
Muscular strength has to do with how strong your muscles get during physical activity.
Flexibility refers to the absolute range of movement in a joint or series of joints, and length in muscles that cross the joints to induce a bending movement or motion. It can be achieved through stretching which is fitness.
SO the correct answer is B.) bone density