Final answer:
Jacob Riis's passage refers to the dangers of truancy for young boys in the slums, illustrating the negative influence of the environment on children without proper guidance or safe play areas.
Step-by-step explanation:
Jacob Riis was referring to the negative influences that the rough urban environment of the slums had on the youth, especially in the absence of proper guidance and play areas. The correct answer to what Riis was referring to in the passage is D) the dangers of truancy for young boys in the slums. Riis's observations, as a social reformer and documentary photographer, revolved around the harsh realities of urban life, and his works, such as How the Other Half Lives, brought awareness to the living conditions of the urban poor. He pointed out that with no positive guidance and engaging activities, children were likely to fall into idle and potentially harmful behaviors. His work served to highlight the struggles of the urban underprivileged, including the lack of safe and healthful play environments, further complicating their already challenging socioeconomic conditions.