Determining the discharge in a stream is an important task in hydraulic engineering. The discharge represents the volume of water flowing through a given section of the stream per unit of time. Several factors need to be considered to accurately determine the discharge, and the most crucial factor among them is the stage at the section.

Stage at the Section:
- The stage at a section refers to the water level or elevation of the water surface at that particular location.
- It is typically measured using a staff gauge or a similar instrument.
- The stage is an essential parameter because it directly affects the cross-sectional area of flow in the stream.
- As the stage increases or decreases, the cross-sectional area of flow also changes accordingly.
- Therefore, to accurately calculate the discharge, it is crucial to know the stage at the section.

Other Factors:
Although the stage at the section is the most important factor in determining the discharge, other data are also required to make precise calculations. These include:

a) Current Meter Readings at Various Verticals:
- Current meter readings provide information about the velocity of the water at different verticals in the stream.
- By taking readings at various verticals, the variation in velocity across the stream can be determined.
- This data is essential for calculating the average velocity and subsequently the discharge.

b) Slope of the Water Surface:
- The slope of the water surface represents the change in elevation of the water surface over a particular distance.
- It is necessary to consider the slope because it affects the flow velocity and the cross-sectional area of flow.
- The slope can be determined by measuring the elevation at two points along the stream and calculating the difference in their heights.

c) Surface Velocity at Various Sections:
- Surface velocity readings provide information about the speed of the water near the surface at different sections of the stream.
- These readings are helpful in estimating the average velocity and discharge.

Conclusion:
While the current meter readings, slope of the water surface, and surface velocity at various sections provide valuable data for calculating the discharge, the stage at the section is the most crucial parameter. Without knowing the stage, it is not possible to accurately determine the cross-sectional area of flow and hence the discharge. Therefore, among the given options, option 'C' (Stage at the section) is the correct answer.