The correct answer is A. Leaping weir.
A leaping weir is a type of weir that is designed to allow excess storm water to flow over it without causing flooding. It does this by creating a series of small waterfalls that dissipate the energy of the water and prevent it from eroding the banks of the stream.
An overflow weir is a type of weir that is designed to allow excess water to flow over it when the water level in the stream reaches a certain point. It does this by creating a small opening in the weir that allows the water to flow through.
A siphon spillway is a type of spillway that is designed to allow excess water to flow over it without creating a large amount of turbulence. It does this by using a siphon to create a vacuum that draws the water over the spillway.
None of these options are the most effective arrangement for diverting excess storm water into a natural drainage. The most effective arrangement is a leaping weir.