Correct answer is (a) -4

Best explanation: A flow obeys conservation of mass if \(\frac{\partial v_x}{\partial x}+\frac{\partial v_y}{\partial y}\)=0. Comparing v=4x+ky with v=vx+vy, we get vx=4x and vy=ky.

Using the conservation of mass, we get \(\frac{\partial (4x)}{\partial x}+\frac{\partial (ky)}{\partial y}\)=0

4+k=0

K=-4.