The code does not permit the use of which material to support drainage piping in a trench?

The correct answer reflects an understanding of the materials suitable for supporting drainage piping in a trench. Blocks, which typically refer to concrete or masonry blocks, are not ideal for this purpose as they can create uneven support and may not allow for proper drainage and settlement. Using blocks can lead to issues such as stress on the pipes, potential cracking, or leaks due to improper alignment.

In contrast, materials like gravel and sand are preferred because they provide stable support and facilitate drainage around the pipes. These materials can prevent sediment buildup, thereby ensuring that the drainage system operates effectively. Additionally, clay soil is generally considered unsuitable for supporting drainage piping due to its tendency to retain water, leading to increased pressure on the pipes and a higher likelihood of failure. Thus, the use of blocks for supporting drainage piping is not permitted by the code due to these concerns about stability and effective drainage.