In which situation should the contractor ensure that the existing drainage system is maintained until the new system is functional?

The situation in which the contractor must maintain the existing drainage system until the new system becomes functional is during stage construction while installing drainage. This is critical to prevent flooding or water accumulation, which could occur if the existing system is not properly maintained. Stage construction often involves building or modifying sections of the drainage system in phases, which means that at any point, there may be partial functionality. Ensuring that the existing system remains operational until the new system is fully connected and functional is essential to protect both the construction site and surrounding areas from water issues. This approach helps manage stormwater effectively and avoid potential damage to the work site or surrounding infrastructure.

In contrast, starting a new construction project or conducting road repairs might not always necessitate the immediate maintenance of an existing drainage system, as the conditions can vary widely based on project scope and needs. Additionally, after completing a previous drainage system installation, the focus would shift towards ensuring the newly installed system is operational, rather than maintaining an existing one. Thus, option B is the most appropriate scenario for this reason.