Final answer:
The melon is the structure used by marine mammals like dolphins to form echolocation waves, which they use to navigate and locate prey.
Step-by-step explanation:
The structure that aids in forming echolocation waves is the melon. The melon is found in certain marine mammals, like dolphins, and is a mass of adipose tissue found in the forehead of the animal. This specialized tissue is used to focus and modulate the sound waves the animal produces. When a dolphin or similar animal emits sound waves, these waves travel through the water, bounce off objects, and return to the animal, which then uses the echoes to form a "picture" of its surroundings. This process of echolocation allows these animals to navigate and locate prey even in murky or dark waters.