Answer: C) Unscrew one light. If the other lights turn off, it's a series circuit.
Explanation:
In a series circuit, all the components in the same path. The current in the entire circuit is same. The voltage drop across each component is different. If one component does not work, the entire circuit breaks and other components also stop working. On the other hand, in a parallel circuit, voltage drop across all the components is equal. Trouble in one component does not effect other components.
Hence, to check whether the lights are connected in series or in parallel, one light should be unplugged and if the other lights still work, it means they are connected in parallel otherwise in series. Thus, correct option is C) Unscrew one light. If the other lights turn off, it's a series circuit.