Concept: Human blood is a connective tissue that is present in liquid form. It performs various functions of carrying gasses and nutrients throughout the body. Some major blood cells are RBC, WBC, and platelets.

Red blood cells are red in color because of the presence of the pigment called hemoglobin. It is made of iron molecules and is used for the transfer of oxygen throughout the body. WBC is a category of blood corpuscles that defend the body from various infections. Platelets are the blood cells that are required for the clotting of blood in case of any injury or wound.

Hence, the correct answer is option D.