To establish that two lines \( l \) and \( n \) are perpendicular, it is necessary to demonstrate that the slopes of these lines are opposite reciprocals of each other. Given the options provided in the context of proving this, the most relevant statement is:
3. The slope of line \( l \) is \( BC/CA \) and the slope of line \( n \) is \( DE/AD \).
This option identifies the slopes of the two lines, which is the first step in showing that they are opposite reciprocals. Option 4 could also be relevant, as it suggests a relationship between the two slopes, but option 3 is fundamental in defining the slopes initially.