Answer
Parallel.
Work Step by Step
When a light bulb burns out, there is a break in the circuit, so no current runs through the connecting wires, as if the light was a switch that is turned off.
If the other light bulb was connected in a series, there would be no current running through it, so even if it is in good working order, it would not produce light.
We occasionally see cars coming toward us with one working headlight, so the wiring of their headlights must be in parallel.
