For a flow to be physically possible it must primarily satisfy which equation?

(a) Equation of conservation of energy

(b) Equation of conservation of mass or continuity equation

(c) Equation of conservation of momentum

(d) None of the mentioned

The question was asked by my college director while I was bunking the class.

My enquiry is from Continuity Equation in Two and Three Dimensions in section Kinematics of Flow and Ideal Flow of Fluid Mechanics