Concept:Mammals rely on specialised internal organs called lungs for gas exchange.
Explanation:The lungs are the primary respiratory organs in mammals.
They are located in the thoracic cavity, one on each side of the heart.
During breathing, oxygen is inhaled into the lungs.
Oxygen passes into the blood while carbon dioxide moves from the blood into the lungs.
This carbon dioxide is then exhaled.
The skin is a respiratory organ in some amphibians, but not in mammals.
The mouth only forms part of the respiratory passage, not a gas-exchange organ.
The heart pumps blood but does not perform respiration.
Therefore, among the given options, the lungs are the correct respiratory organ in mammals.
Answer:D. lungs