condition for compressible flow in terms of vector calculus

Question

For an incompressible flow field, $\vec{V}$ , which one of the following conditions must be satisfied?                                          

      (a) $\triangledown .\vec{V=0}$                                                          (b) $\triangledown \times \vec{V=0}$

      (c) $\left ( \vec{V.\triangledown } \right )\vec{V}=0$                                                     (d) $\frac{\partial \vec{V}}{\partial t}+\left ( \vec{V}.\triangledown \right )\vec{V}=0$