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

Concept:

P-N junction is defined as the boundary between two semiconductor material types which is P-type and N-type. There are three biasing conditions and these conditions are based on the voltage applied. The basing conditions are shown below;

1. Zero bias - Zero bias is defined as the condition in which no external voltage is applied externally to the P-N junction.
2. Forward bias - In the positive terminal the battery is connected with P-type and the negative terminal of the battery is connected with N-type.
3. Reversed bias - In the positive terminal the battery is connected with N-type and the negative terminal of the battery is connected with P-type.​

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Explanation:

When we see in the circuit a) the currents are flowing in the same direction. Therefore, its potential drop is equal.

As we can see in circuit b) the currents are opposite in direction. Therefore, its potential drop is not equal.

and in circuit c) the currents are flowing in the same direction. Therefore, its potential drop is equal.

Hence, option 1) is the correct answer.