(b) Co-ordinates of the point are (x, y)
$\therefore\ \ $Distance of point from origin,
$r = \sqrt{x^{2} + y^{2},\ }\ V = - kxy$
$E_{x} = - \frac{dV}{dx} = - \frac{d}{dx}( - kxy) = ky$
$E_{y} = - \frac{dV}{dy} = ( - kxy) = kx$
$\therefore\ E = \sqrt{E_{x}^{2} + E_{y}^{2}} = k\sqrt{y^{2} + x^{2}} = kr$
$\therefore\ E \propto r$