Answer:
The verb that best completes the sentence is "interact."
Geographers study how people interact with their environment.
"Interact" is the correct verb in this context because it accurately describes the action of people engaging or communicating with their environment. It shows the dynamic relationship between people and their surroundings.
Other options like "interacted," "will have interacted," and "has interacted" are not appropriate because they do not match the tense and do not convey the ongoing nature of the study. "Interact" is the present tense verb that best fits the sentence.