which one of the following is a necessary condition?

Question

For a continuity equation given $\triangledown .\vec{v}=0$ to be valid, $\vec{v}$where is the velocity vector, which one of the following is a necessary condition?                                                                   

      (a) Steady flow

      (b) Irrotational flow

      (c) Inviscid flow

      (d) Incompressible flow