Answer: A. No current will flow in the circuit.
Explanation:
The Figure 1 illustrates the equivalent circuit at only switch S1 closed. That is a series circuit with two diodes, a bulb, and a battery.
Suppose that there is a current flowing in the circuit. A current flowing in the circuit implies that D1 is on. If D1 is on, there should be 0.7 V between anode and cathode (see Figure 2 left). However, there is not possible a low voltage on the top and a high voltage at the bottom of D1 (see Figure 2 right). Then, it is not viable 0.7 V between anode and cathode of diode D1. Therefore, D1 is off.
Now, we already know that this is a series circuit with the diode D1 off, so we can conclude that no current will flow in the circuit.
Correct answer is A. No current will flow in the circuit.
There isn't current flowing in the circuit next the light bulb won't light up. But other things happen, for example, the high voltage goes to the light bulb at only switch S1 closed. we can say that option B. is partially wrong and options C. is wrong
Finally, there are no resistances in the circuit. So, option D. is not right
