Answer : 4. "She switched off the light before sleeping."
Explanation :

In Active Voice, a sentence emphasizes the subject, performing an action.
In Passive Voice, a sentence emphasizes the action or the object of the sentence.
The given sentence is in the passive voice and 'The light' is the subject and 'her' is the object.
When we convert this sentence into active voice, the subject 'The light' of the passive voice becomes the object, the object 'her' becomes the subject 'she'.
The passive format "was + V3 (switched)" should be changed into the active format "switched".
This is the active and passive voice rule for the past simple tense.