To have an equal potential drop across two identical p-n junction diodes in a series circuit, both diodes must be in the exact same biasing condition (i.e., both forward-biased or both reverse-biased). If one diode is forward-biased and the other is reverse-biased, their resistances will differ significantly, leading to unequal potential drops. Although the specific circuit diagrams are missing from the text, the correct option implies that circuits (a) and (c) feature symmetric biasing for both diodes, while circuit (b) does not.