A fall which maintains its depth, is

A fall is a structure constructed across a canal to lower the water level and dissipate surplus energy caused by a difference in bed levels.

A low weir fall is specifically designed to maintain the depth of flow in the canal. It allows water to flow smoothly over a broad-crested weir while keeping the upstream depth nearly constant.

This type of fall helps in energy dissipation and prevents scouring and erosion downstream by stabilizing the flow conditions.

Trapezoidal notch fall and rectangular notch fall are primarily used for measuring discharge and controlling flow rather than maintaining depth.

Hence, among the given options, the fall that maintains its depth is the low weir fall.

Therefore, the correct answer as per exam and technical standards is Option B: A low weir fall.