The value of $\lambda$ for a possible flow field involving an incompressible fluid is

Question

The velocity components in the x and y directions are given by   U= $\lambda$xy³ – x²y, $v=xy^{2}-\frac{3}{4}y^{4}.$ . The value of $\lambda$ for a possible flow field involving an incompressible fluid is                                                                       

      (a) -$\frac{3}{4}$                              (b)-$\frac{4}{3}$                           (c)$\frac{4}{3}$                               (d) 3