Pipe material most commonly adopted for water-distribution mains in traditional municipal systems:

Introduction / Context:
Municipal water-distribution mains historically used cast iron due to its pressure capacity, stiffness, and established jointing methods. While modern systems often use ductile iron or plastics, older specifications and many current networks still cite cast/ductile iron as standard.

Given Data / Assumptions:

• Pressurized buried distribution mains carrying potable water.
• Conventional civil engineering practice context.

Concept / Approach:
C.I. offered a balance of strength, durability, and ease of joining with spigot-and-socket or mechanical joints. R.C.C. is more common for large sewers or gravity mains; G.I. is used in smaller service connections; Hume pipes refer to concrete pipes more suited for non-pressurized or low-pressure applications.

Step-by-Step Solution:

Verification / Alternative check:
Many city asset registries list extensive C.I./ductile iron networks, evidencing long-term adoption.

Why Other Options Are Wrong:

• R.C.C./Hume pipes are better for gravity sewers and culverts.
• G.I. is typically for small diameters and limited lengths due to corrosion concerns.
• Asbestos cement (e) is obsolete for health and durability reasons in many regions.

Common Pitfalls:
Assuming gravity pipe materials are interchangeable with pressure mains; pressure class and joints matter greatly.

Final Answer:
Cast iron (C.I.) pipes