For an incompressible fluid, the discharge is usually expressed as the: 

Concept:

Discharge or Rate of flow:

• It is defined as the amount of fluid flowing through a section of a channel or a pipe per unit of time.
• The discharge may be expressed in terms of the weight of flowing fluid per unit of time or mass of flowing fluid per unit of time or volume of flowing fluid per unit of time through a channel or pipe.
• The units of discharge for liquids are m³/s or litres/s.
• The units of discharge for gases are N/s or kg/s.

Note:

• In the case of an incompressible fluid, the discharge is usually expressed as the volume of flowing fluid per unit of time (volume flow rate of the fluid).
• Whereas, in the case of compressible fluid, the discharge is expressed as the weight of flowing fluid per unit of time.

Let us consider a fluid flowing through a pipe;

A = cross-sectional area of the pipe, V = average velocity of flowing fluid through the pipe

\(v\) = specific volume = \(\frac{1}{\rho}\)

Discharge, \(Q = AV=mv\) (m³/s)

or \(m=\frac{AV}{v}\)\(=\rho{A}{V}\) (kg/s)

or \(mg = \rho{g}AV\)

\(W=\rho{g}AV\) (N/s)

Thus, option (4) is the correct answer.