Pipe materials that are the least susceptible to corrosion by acids formed from gases generated in sewers is:

The correct answer is based on the characteristics of various pipe materials in relation to their susceptibility to corrosion from acidic conditions that can arise from gases in sewer environments.

Vitrified clay pipe (VCP) is known for its durability and resistance to corrosion, particularly in conditions where acidic compounds are present. The vitrification process creates a glass-like surface that is less reactive with harsh chemicals, making it more suitable for sewer applications where such gases can generate acids. Its low permeability also helps to resist the infiltration of corrosive substances, further enhancing its longevity and performance in sewer systems.

In contrast, the other materials listed, like ACP (Aluminum Composite Pipe), CIP (Cast Iron Pipe), and RCP (Reinforced Concrete Pipe), have varying degrees of susceptibility to corrosion. These materials may react with the acidic compounds generated in sewer environments, leading to deterioration over time and potentially resulting in structural failures or leaks.

Thus, the selection of VCP is justified due to its specific properties that offer superior resistance to corrosion, making it the most appropriate choice for applications exposed to acidic conditions derived from sewer gases.