Final answer:
The least likely part of a home built before 1978 to contain lead is the roof.
Step-by-step explanation:
The part of a home built before 1978 that would be the least likely to contain lead is the roof. According to the information provided, the percentage of homes built from 1960-1977 that have lead-based paints is only 24%. In contrast, the graph shows that the majority of homes built before 1940 (87%) and between 1940 and 1959 (69%) have lead-based paints. Among the parts of a home built before 1978, plumbing would be the least likely to contain lead, specifically when compared to the roof, walls, and windows. Reports by the U.S. Environmental Protection Agency indicate that lead-based paints were predominantly used during this time, especially in homes built before 1940. However, in plumbing, the main concern would be with the water pipes that could potentially be made of lead, but if the rest of the plumbing is iron, as stated in an exercise, then the pipes would have already been of more concern for lead exposure than other parts of the home. Therefore, with respect to paint, which is the most common source of lead in older homes, the plumbing fixtures themselves, not involving the pipes, would be less likely to contain lead.