Assumption in Psychological Argument on Nature's Benefits

In a recent article, a psychologist argues that frequent exposure to nature leads to improved mental well-being. The psychologist cites several studies showing a correlation between time spent outdoors and reduced anxiety and depression symptoms. Based on these findings, the psychologist concludes that increasing green spaces in urban areas will enhance the mental health of city dwellers.

This conclusion, however, relies on several assumptions. Which of the following statements is an assumption that the psychologist's argument depends on?