Respiration is a word that is used in several contexts. In all contexts, it involves the process by which body avails the key element for survival, oxygen.
Inhalation (C) is the physical process of breathing, by which air is taken in. It is initiated by the distension of diaphragm, which enlarges the thoracic cavity and this enables air to enter.
Internal respiration (B) is the process by which the oxygen taken in through the lungs and transported by the blood is exchanged to various cells. This occurs in the capillaries throughout the body.
Cellular respiration (D) is the molecular level process by with redox equivalents synthesized by glycolysis and Krebs cycle is oxidized to generate ATP within the mitochondria.
Therefore, the options C, B and D are incorrect.
External respiration (A) is the exchange of oxygen from air into blood via the walls of the alveoli. Alveoli are thin sacs present at the end of bronchioles (within the lungs) that are highly innervated by blood vessels.
Therefore, option (A) is correct.