Introduction:
The energy dissipation at the toe of the spillway refers to the reduction or dissipation of the energy of the water flow as it leaves the spillway and enters the downstream river bed. This is important to prevent erosion and other negative impacts on the river bed and surrounding areas. The use of hydraulic jump is an effective method to achieve energy dissipation and prevent excessive turbulence and erosion.

Explanation:
Sloping apron below the downstream river bed:
The use of a sloping apron below the downstream river bed is an effective method to dissipate the energy of the water flow. The sloping apron provides a smooth transition for the water from the spillway to the river bed, reducing the velocity and impact of the water flow. As the water flows down the sloping apron, it spreads out and loses its energy through friction and turbulence. This helps to prevent erosion and other negative impacts on the river bed.

Roller bucket:
A roller bucket is a type of spillway structure that uses a series of rolling buckets to dissipate the energy of the water flow. The buckets are designed to rotate as the water flows over them, creating turbulence and reducing the velocity of the water. This helps to dissipate the energy of the water flow and prevent erosion. However, the use of a roller bucket alone may not be sufficient to effectively dissipate the energy at the toe of the spillway.

Ski-jump bucket:
A ski-jump bucket is another type of spillway structure that uses a curved or sloping surface to create a hydraulic jump. The hydraulic jump occurs when the velocity of the water flow decreases rapidly, causing a sudden increase in the water depth. This rapid change in flow conditions dissipates the energy of the water flow and reduces its velocity. The ski-jump bucket is designed to create and control this hydraulic jump, thereby effectively dissipating the energy at the toe of the spillway.

Conclusion:
In conclusion, the energy dissipation at the toe of the spillway is primarily affected by the use of a sloping apron below the downstream river bed. While roller buckets and ski-jump buckets can also contribute to energy dissipation, they may not be as effective as a sloping apron in reducing the velocity and impact of the water flow. Therefore, the correct answer is option 'C'.