Answer:
D. Acoustics
Step-by-step explanation:
Environmental scientists have some difficulty identifying gray wolves, because wolves tend to move away from humans. For this reason, scientists have developed a technique for identifying wolves by their individual howls. This technique is acoustics.
When they occupy a territory, they have a natural tendency to signal to other wolves. Using acoustics, scientists can record a wolf's howl and release this recording in the forest, when it does, other wolves can respond to the recording's howl and this will allow scientists to approach the territory where the gray wolves are .