To solve this problem, we need to analyze the potential drop across the p-n junctions in each circuit.$\\$In a p-n junction, the potential drop depends on the biasing of the junction:$\\$• Forward bias reduces the potential barrier, allowing current to flow easily.$\\$• Reverse bias increases the potential barrier, preventing current flow.$\\$Let's analyze each circuit:$\\$Circuit (a):$\\$• Both p-n junctions are forward biased.$\\$• The potential drop across each junction is equal because they are identical and equally biased.$\\$Circuit (b):$\\$• The first junction (p-n) is forward biased, while the second junction (n-p) is reverse biased.$\\$• The potential drops are not equal due to different biasing.$\\$Circuit (c):$\\$• Both n-p junctions are reverse biased.$\\$• The potential drop across each junction is equal because they are identical and equally biased.$\\$Conclusion:$\\$The potential drop across the two p-n junctions is equal in circuits (a) and (c).$\\$Therefore, the correct option is 4: Both circuits (a) and (c).