In the given circuits (a), (b) and (c), the potential drop across the two $p-n$ junctions are equal in:

In circuit(a), both p-n junctions are in forward bias, hence current will flow and potential drop will be equal.

In circuit(c), both p-n junctions are in reverse bias and hence negligible current will flow through the circuit and potential drop will be equal.

In circuit(b), one p-n junctions is in reverse bias and one in forward bias. So, they will show different resistances. Also, negligible current will flow through the circuit. Hence, potential drop will be different.