Explanation:

The correct answer is option 'B' - the bulb glows only in a closed circuit.

Open Circuit:
- An open circuit is a circuit where there is a break or interruption in the flow of electric current.
- In an open circuit, the electrical path is incomplete, and therefore, the current cannot flow through the circuit.
- When the circuit is open, there is no complete loop for the current to flow from the power source to the bulb and back.
- In this case, the bulb does not receive any current, and hence, it does not glow.

Closed Circuit:
- A closed circuit is a circuit where there is a complete path for the electric current to flow.
- In a closed circuit, the electrical path is continuous, and therefore, the current can flow through the circuit.
- When the circuit is closed, there is a complete loop for the current to flow from the power source to the bulb and back.
- In this case, the bulb receives the current, and it gets energized, causing it to glow.

Incorrect Options:
a) Open circuit: The bulb does not glow in an open circuit as there is no complete path for the current to flow.
c) In both circuits: The bulb does not glow in an open circuit, so it cannot glow in both circuits.
d) Open circuit if bulb is not fused: The condition of the bulb being fused or not does not affect whether it glows in an open circuit. In both cases, the bulb does not glow in an open circuit.

Conclusion:
The bulb glows only in a closed circuit because it requires a complete loop for the electric current to flow through it. In an open circuit, there is a break in the path, and therefore, the current cannot reach the bulb, causing it not to glow.