For a bulb to glow the circuit must be complete. Option (a) states that bulb A will glow but the bulb A will not glow until the circuit completes. And if the circuit completes, all bulbs will glow together because all are connected to each other. The circuit process in following steps:
- Current leaves from the positive terminal.
- Current enters the bulb A, but the circuit is not complete, hence the bulb does not glow.
- Current enters the bulb B, but the circuit is not complete, hence the bulb does not glow.
- Current enters the bulb C, but the circuit is still not complete hence the bulb does not glow.
- But when the current enters the negative terminal the circuit completes and all the bulb will glow at once. The simple logic here is that all bulb will help to complete the circuit, that is why the current will flow through them at the same time and hence they will glow together