The total water requirement of a city is generally assessed on the basis of:

a) Maximum hourly demand
The maximum hourly demand refers to the peak water usage during a specific hour of the day. This demand is usually higher than the average demand and is used to determine the capacity of the water supply infrastructure to meet the needs of the city during peak times.

b) Maximum daily demand
The maximum daily demand represents the highest water usage throughout a 24-hour period. This demand takes into account the fluctuation in water consumption patterns throughout the day. It is important to consider the maximum daily demand to ensure that the water supply system can adequately meet the needs of the city even during periods of high demand.

Fire demand
Fire demand refers to the amount of water required to combat fires in the city. This demand is typically calculated based on the number and size of buildings in the city, as well as fire safety regulations. Fire demand is an essential factor to consider when assessing the total water requirement of a city, as it ensures that the water supply system can provide sufficient water pressure and volume to extinguish fires effectively.

c) Average daily demand
The average daily demand represents the typical water usage over a 24-hour period. It takes into account the regular consumption patterns of the city's population and is useful for long-term planning and infrastructure design. However, it may not account for the occasional spikes in demand or the need for firefighting capabilities.

d) Greater of (a) and (b)
The correct answer is option 'D' because the total water requirement of a city should be assessed based on the greater of the maximum hourly demand and the maximum daily demand. This approach ensures that the water supply system can handle both peak usage periods and sustained high demand throughout the day. By considering both factors, the water infrastructure can be designed to meet the city's needs adequately and ensure the availability of water for firefighting purposes.

In conclusion, the total water requirement of a city should be assessed based on the greater of the maximum hourly demand and the maximum daily demand, taking into account the need for firefighting capabilities. This approach ensures that the water supply system can handle peak usage periods, sustained high demand, and provide sufficient water for fire protection.