Answer: The right answers are 3) It indicates that the gangsters are looking for vulnerable people and 5) It supports the idea that the gangsters are involved in criminal activities.
Explanation: Just to elaborate a little on the answer, it can be added that the word "prowl" suggests that the person, or the animal, is moving cunningly and quietly in order not to be seen or heard, as when an animal is hunting for a prey or a burglar is trying to break into someone's house. These gangsters prawl danger areas, such as train tops, looking for people who are sleeping, and, therefore, are defenseless, or, at least, not prepared to protect themselves. For those reasons, the meaning of the word "prowl" support the author's purpose in this excerpt, which is to indicate that the members of this gang are dangerous, are involved in criminal activities (nobody should prowl the train tops), and look for vulnerable people in order to attack them, and because the police in Chiapas treat them better, they settle there—rather than in their home countries—after being expelled from the US.