A string of lights in a series circuit is not working. Which of the following is most likely the cause? A light bulb burned out and “opened” the circuit.
A light bulb burned out and “closed” the circuit.
One of the light bulbs is dimmer than the others.
One of the light bulbs is brighter than the others.

B. a light bulb burned out and "closed" the circuit.

Explanation:

Unscrew one bulb. If the other bulbs turn off, it is a series circuit. A light bulb glows when electrons flow through it. As the electron flow increases, the brightness increases.

When lights are attached in parallel, each light is on its own circuit to the power source. If one filament burns out, it has no effect on the remaining lights, as they each continue to be in a closed circuit with the power source.