For the potential drop across two diodes to be equal, both must be in the same biasing state (either both forward biased or both reverse biased) and connected in series.

In circuits (a) and (c), both diodes face the same direction relative to the battery, so they share identical bias conditions and present equal resistance.

Since the two diodes are in series and carry the same current, equal resistance gives equal potential drop across each junction.

In circuit (b), the diodes are oppositely oriented, so one is forward biased while the other is reverse biased, leading to unequal drops.