The correct answer is option 'B' - when the inflow is into a reservoir with an uncontrolled outlet.

When routing a flood through a reach, it is important to understand the concept of inflow and outflow hydrographs. Inflow hydrograph represents the flow entering a reach, while outflow hydrograph represents the flow leaving the reach. Flood routing refers to the process of determining how the inflow hydrograph is transformed as it travels through the reach and is eventually discharged as the outflow hydrograph.

To understand why the point of intersection of inflow and outflow hydrographs coincides with the peak of the outflow hydrograph in the case of inflow into a reservoir with an uncontrolled outlet, let's look at the characteristics of both hydrographs:

1. Inflow Hydrograph:
- Represents the flow entering the reach, which is influenced by factors such as rainfall intensity, catchment area, and drainage characteristics.
- Typically has a rising limb, a peak, and a recession limb.
- The peak of the inflow hydrograph represents the maximum flow entering the reach.

2. Outflow Hydrograph:
- Represents the flow leaving the reach, which is influenced by the storage capacity and release characteristics of the reach.
- Typically has a rising limb, a peak, and a recession limb.
- The peak of the outflow hydrograph represents the maximum flow leaving the reach.

Explanation:
When the inflow is into a reservoir with an uncontrolled outlet, the reservoir does not have any mechanism to control the outflow. As a result, the outflow hydrograph directly reflects the inflow hydrograph. This means that the peak of the outflow hydrograph will coincide with the peak of the inflow hydrograph.

In this scenario, as the inflow hydrograph rises, the water accumulates in the reservoir until it reaches its maximum storage capacity. Once the reservoir is full, any additional inflow cannot be stored and is immediately discharged through the uncontrolled outlet. Therefore, the outflow hydrograph directly reflects the inflow hydrograph, and their peaks coincide.

It is important to note that this relationship between the inflow and outflow hydrographs holds true only when the inflow is into a reservoir with an uncontrolled outlet. In other cases, such as channel routing or reservoir routing with controlled outlets, the point of intersection of inflow and outflow hydrographs may not coincide with the peak of the outflow hydrograph.