Physics: Principles with Applications (7th Edition)

The electric field at the lower-right-hand corner is the vector sum of the electric fields caused by the charges at the other three corners. The two positive charges in the lower left and upper right create equal-magnitude E fields pointing in the positive x direction and in the negative y direction, respectively. These add to produce an electric field in the direction of choice D. By the Pythagorean theorem, this field has a strength of $\sqrt{2}$ times the magnitude of the E field due to just one of the charges. The E field created by the remaining negative charge points in the direction of choice B. Because the negative charge is $\sqrt{2}$ times farther away than either of the positive charges, the magnitude of its electric field is insufficient to cancel the field from the 2 positive charges. The net field points in the direction of (d).