What type of pipe typically forms tubercles?

Tubercles are small, rounded projections that can form on the interior surfaces of pipes due to corrosion or sediment buildup, and they are primarily associated with certain types of materials used in piping systems. Ductile iron pipes are particularly susceptible to tuberculation, especially in environments where water quality is not optimal, such as in the presence of high levels of certain bacteria or in waters that encourage corrosion.

Ductile iron has a relatively rough surface compared to other pipe materials, which can provide a suitable surface for corrosion processes to initiate and lead to tubercle formation. These tubercles can reduce the flow capacity of the pipe and can affect water quality by harboring bacteria and other contaminants.

On the other hand, materials like PVC and concrete do not typically develop tubercles because they are not prone to corrosion in the same way that ductile iron is. Steel can corrode and form rust, but it does not create tubercles in the same manner or to the same extent as ductile iron. Thus, ductile iron is clearly identified as the material most associated with the formation of tubercles in pipe systems, making it the correct answer for this question.