Some equipotential surfaces are shown in figure. The electric field strength is

100 V/m along x-axis

100 V/m along y-axis

400 V/m at an angle 120° with x-axis

$\frac{400}{\sqrt{3}}\mathrm{V}/\mathrm{m}\text{ an angle }{120}^{\circ }\text{ with }\mathrm{x}\text{-axis }$