**Explanation:**

River training works refer to various engineering interventions and techniques used to modify and control the flow of rivers. These works are aimed at achieving specific objectives related to discharge, depth, and sediment management. Therefore, the correct answer is option 'D', which states that river training works may be classified as training for discharge, depth, and sediment.

**Training for Discharge:**
- River training works can be designed and implemented to regulate the discharge or flow of water in a river channel.
- This involves the construction of structures such as barrages, weirs, dams, and embankments that control the amount of water passing through the channel.
- By regulating the discharge, river training works can help prevent flooding, ensure a consistent water supply for irrigation and other uses, and manage the overall water balance in a river system.

**Training for Depth:**
- River training works can also be focused on maintaining a desired depth of the river channel.
- This may involve dredging or excavation to remove excessive sediment or debris that can cause the channel to become shallow.
- Additionally, structures such as groynes or spurs can be constructed to redirect the flow of water and prevent sediment deposition in certain areas, thereby maintaining the desired depth.

**Training for Sediment:**
- Sediment management is another important aspect of river training works.
- Sediment deposition can lead to channel aggradation, which can cause flooding and hinder navigation.
- River training works may involve the construction of sediment traps, sediment bypasses, or sediment removal structures to control the movement and accumulation of sediment in the river channel.

Overall, river training works encompass a range of engineering interventions and techniques that address the discharge, depth, and sediment management in river systems. These works aim to improve the overall functioning of the river, mitigate the risks of flooding, maintain navigation routes, and ensure sustainable water management.