Correct Option (d):
Most insects respire using a tracheal system. This system consists of a network of air-filled tubes called tracheae that branch throughout the insect's body. These tracheae open to the exterior through small pores known as spiracles, located on the body surface. Oxygen directly diffuses through this network to reach individual cells and tissues, facilitating gas exchange without relying on the circulatory system for oxygen transport.
Incorrect Options:
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Through skin: Insects do not respire cutaneously (through their skin). Cutaneous respiration is observed in some amphibians and other organisms.
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Through gills: Gills are respiratory organs adapted for aquatic environments, used by fish and some other aquatic animals. Terrestrial insects do not possess gills for respiration.
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By lungs: Insects do not have lungs. Lungs are complex respiratory organs found in most terrestrial vertebrates, including mammals, birds, reptiles, and amphibians.