Which type of drainage structure is used where no surface water enters the system?

The correct answer in this context is manholes. Manholes serve as access points in a drainage system for maintenance and inspection of underground utility lines, including sanitary sewer systems and storm drains. They are designed to provide a means to enter the drainage structure without allowing surface water to infiltrate the system directly.

Manholes primarily connect different sections of pipes and allow for the flow of wastewater without being designed to handle the additional influx of surface water, which makes them suitable for situations where surface water does not enter the drainage system. Proper construction and sealing of manholes help prevent the entry of surface runoff.

The other options represent different structures that are typically intended to manage or collect surface water. Drain inlets and catch basins are designed to capture and direct surface water runoff into the drainage system, thereby intentionally allowing surface water to enter. Stormwater ponds are designed to collect and manage stormwater runoff as well, serving as temporary storage to control flooding and improve water quality. These structures operate under different principles compared to manholes, which focus more on accessibility and maintenance of existing drainage systems.