Design Period:
Design period refers to the expected lifespan or duration for which a particular unit or system is designed to operate effectively. It is an important consideration in engineering design as it helps determine the appropriate specifications and requirements for the unit.

Maximizing Design Period:
To maximize the design period, the unit or system should be designed to withstand the expected operational conditions and remain functional for a longer duration without significant maintenance or replacement. This can be achieved by using durable materials, implementing robust design features, and considering potential future changes or expansions.

Analysis of the Options:
a) Distribution System:
A distribution system refers to the infrastructure used to distribute water from the source (e.g., water treatment plant) to the consumers. While it is important to design a distribution system that can effectively transport water to various locations, the design period may not be the primary concern. The design of the distribution system is more focused on optimizing flow rates, pressure, and minimizing losses.

b) Demand Reservoir:
A demand reservoir, also known as a storage reservoir, is a component of the water supply system that stores water to meet peak demand periods. The design period of a demand reservoir is typically maximized by considering factors such as the projected increase in water demand, potential changes in population, and the expected lifespan of the reservoir structure. By accurately estimating these factors, the design period of the demand reservoir can be extended.

c) Water Treatment Unit:
A water treatment unit is responsible for purifying and treating raw water to make it suitable for consumption or specific industrial processes. The design period of a water treatment unit is influenced by factors such as the quality of the raw water, the treatment processes employed, and the expected changes in water quality or regulations. Maximizing the design period of a water treatment unit involves considering the long-term effectiveness of the treatment processes, potential changes in raw water quality, and the durability of the equipment used.

d) Pipe Mains:
Pipe mains are the primary pipelines used to transport water from the source or treatment plant to the distribution system. The design period of pipe mains is typically influenced by factors such as the expected changes in water demand, the type of pipe material used, and the potential for future expansions or modifications. Maximizing the design period of pipe mains involves selecting durable pipe materials, considering potential changes in demand, and designing for future growth.

Conclusion:
Among the given options, the design period is maximized for the demand reservoir. By accurately estimating the expected increase in water demand, considering potential changes in population, and designing for the expected lifespan of the reservoir structure, the design period of a demand reservoir can be extended.