To answer the question, it is important to understand the concept of rooftop rainwater harvesting and the geographical conditions of the mentioned cities.

Rooftop rainwater harvesting is a method of collecting and storing rainwater that falls on rooftops for later use. It involves collecting rainwater from the rooftops, filtering it to remove any impurities, and storing it in tanks or underground reservoirs for various purposes such as irrigation, household use, or groundwater recharge.

Now, let's analyze the given options and determine why Shillong is the most common practice for rooftop rainwater harvesting:

1. Shillong:
Shillong, the capital city of Meghalaya, is known for its abundant rainfall throughout the year. The city experiences heavy rainfall during the monsoon season, which makes it an ideal location for rooftop rainwater harvesting. The hilly terrain and sloping roofs in Shillong facilitate the easy collection of rainwater. Moreover, due to the scarcity of water in certain areas of the city, the practice of rainwater harvesting has gained significant popularity among the residents.

2. Guwahati:
Guwahati, the largest city in Assam, also receives a substantial amount of rainfall. However, the practice of rooftop rainwater harvesting is not as common as in Shillong. The reasons for this could be the geographical layout of the city, the availability of alternative water sources, or the lack of awareness and promotion of rainwater harvesting techniques.

3. Imphal:
Imphal, the capital city of Manipur, also experiences a good amount of rainfall. However, rooftop rainwater harvesting is not widely practiced here. Similar to Guwahati, the reasons could be related to the geographical conditions, lack of awareness, or the availability of other water sources.

4. Patna:
Patna, the capital city of Bihar, receives moderate rainfall. While rainwater harvesting is not entirely uncommon in Patna, it is not as prevalent as in Shillong. The city's flat terrain and lack of awareness about rainwater harvesting techniques might explain the lower adoption rate.

Therefore, based on the given options, Shillong is the most common practice for rooftop rainwater harvesting. The combination of heavy rainfall, hilly terrain, and increased awareness about the importance of water conservation has made rooftop rainwater harvesting a popular choice among the residents of Shillong.

Shillong gets very much rain. So, the common practice is rain harvesting in that place.