Answer:
Engaging in an ethnography.
Explanation:
Ethnography is a research method used to understand the society or culture. In this method, the researchers analyze the society and culture and keep the records in written form. Ethnography is a type of anthropology study. Ethnography provides in-depth case studies of the culture or society.
In the given case, a criminologist should use the method of ethnography to understand the street gang. Because ethnography will provide them with an in-depth case study of the street gang also.
So, the correct answer is the last option.