The correct choice is (a) 125 V
For explanation I would say: Current through the 18 Ω resistance = \(\frac{90}{18}\) = 5 A
Equivalent resistance of 3 Ω and 7 Ω banks = \(\frac{3 × 6}{3 + 6}\) = 2 Ω
Since, this 2 Ω resistance is in series with 18 Ω resistance, therefore total resistance = 18 + 2 = 20 Ω
This 20 Ω resistance is in parallel with 5 Ω resistance = \(\frac{5 × 20}{5 + 20}\) = 4 Ω
Hence, total resistance of the circuit = 1 + 4 = 5 Ω
Current through this branch = 5 A
∴ Voltage across dc= 5 × 20 = 100 V
Hence current through 5 Ω resistance = \(\frac{100}{5}\) = 20 A
∴ Total current = 20 + 5 = 25 A
Since, total resistance of the circuit is 5 Ω therefore, voltage E = 25 × 5 = 125 V.