The correct answer is: A. Industrial areas.
Drop manholes are generally provided in sewers for industrial areas because they are designed to withstand the heavy traffic and wear and tear that is common in these areas. They are also typically larger than other types of manholes, which makes them easier to access and maintain.
Large towns and cities may also have drop manholes, but they are not as common as in industrial areas. This is because the traffic and wear and tear in these areas is not as severe. Additionally, large towns and cities often have other types of manholes that are better suited for their needs.
Hilly towns and cities may also have drop manholes, but they are not as common as in other types of areas. This is because the terrain in these areas can make it difficult to install and maintain drop manholes. Additionally, the slope of the land can make it difficult for sewage to flow through drop manholes.
Cities in plains are the least likely to have drop manholes. This is because the terrain in these areas is flat, which makes it easy to install and maintain other types of manholes. Additionally, the flat terrain makes it easy for sewage to flow through other types of manholes.