The correct answer is: A. Steel
Steel is a strong, durable material that is not easily corroded. It is also a long-lasting material, making it a good choice for water supply pipes. However, steel is also heavy and brittle, which can make it difficult to install and transport.
Cast iron is a strong, durable material that is also not easily corroded. However, cast iron is more brittle than steel, and it is more susceptible to cracking.
Copper is a strong, durable material that is not easily corroded. It is also a long-lasting material, making it a good choice for water supply pipes. However, copper is more expensive than steel or cast iron.
Reinforced cement concrete is a strong, durable material that is not easily corroded. It is also a long-lasting material, making it a good choice for water supply pipes. However, reinforced cement concrete is more expensive than steel or cast iron, and it is more difficult to install.
In conclusion, steel is the most commonly used material for water supply pipes because it is strong, durable, and not easily corroded. However, it is also heavy and brittle, which can make it difficult to install and transport.