You have to remember that all of the parts of the body have their own functions. Some of them will not make it possible for humans to survive. There are others that human beings can live without but would need to make some drastic changes to address the change.
When it comes to the lung, the smallest active functional unit has to be letter B. This contains the alveoli and the bronchiole. Take note that the alveolus can be associated with the exchange. Letter C may seem like the best answer in the beginning because it is a functional part of the lung but the key here is to find the smallest unit.
A respiratory bronchiolar unit -the smallest functional unit of the lung is the respiratory bronchiolar unit, which contains a respiratory bronchiole and the alveoli associated with it. this unit allows for air conduction and gas exchange. the alveolus is only associated with gas exchange, and the bronchi form part of the conduction system. the bronchopulmonary segment is a functional unit of lung structure, but it is not the smallest unit. bronchopulmonary segments are particularly important in surgical resections of the lung because they represent functional units with connective tissue boundaries and individualized vasculature, including pulmonary and bronchial arteries, pulmonary lymphatics, and pulmonary nerves, all of which follow the air-conducting system of the bronchial tree and its branches.