In the middle of depletion layer of reverse biased p-n junction:

The correct answer is Option (4) → Electric field is maximum

The depletion layer holds fixed ionised donors on the n-side and fixed ionised acceptors on the p-side. The field they produce points from the n-side to the p-side and builds up from zero at the outer edges of the layer to its largest value at the junction, in the middle of the layer. The field is the slope of the potential, \( E = -\frac{dV}{dx} \), so the potential changes fastest across the middle; a reverse bias widens the layer and raises this field.