The correct answer is option 'A', coefficient of discharge.

Explanation:
The coefficient of discharge is a dimensionless quantity that represents the ratio of the actual discharge of a fluid to its theoretical discharge. It is denoted by the symbol Cd.

When a fluid, such as water, is discharged through a jet or orifice, the actual discharge may not be equal to the theoretical discharge due to various factors such as friction, turbulence, and losses in the flow. The coefficient of discharge takes into account these factors and provides a measure of the efficiency of the discharge.

The coefficient of discharge can be calculated using the following formula:

Cd = (Actual Discharge) / (Theoretical Discharge)

The theoretical discharge is the discharge that would occur if the fluid were to flow without any losses or disturbances. It can be calculated using the following formula:

Theoretical Discharge = Cd * (Area of Orifice) * (√(2gH))

Where:
- Cd is the coefficient of discharge
- Area of Orifice is the cross-sectional area of the orifice through which the fluid is discharged
- g is the acceleration due to gravity
- H is the head or height of the fluid above the orifice

By comparing the actual discharge with the theoretical discharge, we can determine the efficiency of the discharge. If the actual discharge is lower than the theoretical discharge, it indicates that there are losses in the flow, such as friction or turbulence. On the other hand, if the actual discharge is higher than the theoretical discharge, it suggests that there may be other factors affecting the flow, such as a higher pressure or flow rate.

In practical applications, it is important to determine the coefficient of discharge in order to accurately predict and control the flow of fluids. This information is particularly useful in industries such as water supply, irrigation, and hydraulics, where the efficient use of water is crucial.

In conclusion, the coefficient of discharge is the ratio of the actual discharge of a fluid to its theoretical discharge. It provides a measure of the efficiency of the discharge and is an important parameter in fluid flow calculations.