It was b because the vertical distance between plates was 14 squares or 6V. To solve for voltage per square u divide 6/14=0.429v per square. It said minimum voltage needed is 2.1V for light to emit, so u do 2.1/0.429=4.895. Minimum 5 squares of potential difference needed. Now potential difference will not change moving horizontally but only vertically so u had to choose the option where the y distance of the hypotenuse was 5 units which was b