The correct answer is D. The lights were switched on by the watchman before it began to grow dark.
The sentence “The watchman switched on the lights before it began to grow dark” is in the past tense. This means that the action of switching on the lights happened before the time of speaking. The sentence also uses the active voice, which means that the subject of the sentence (the watchman) is doing the action (switching on the lights).
Option A is incorrect because it uses the passive voice. In the passive voice, the subject of the sentence is acted upon by the verb. In this case, the lights are being switched on, but the sentence does not say who is switching them on.
Option B is incorrect because it uses the present tense. The sentence is about an action that happened in the past, so it should be in the past tense.
Option C is incorrect because it uses the past participle “being switched on”. The past participle is used to form the present perfect tense and the past perfect tense. In this case, the sentence is about an action that happened in the past, so it should not use the past participle.