Explanation:

- Original sentence: "He had finished the task, before he left."

- "He" completed the task before departing.

- Verb tense shows the task's completion precedes leaving.

- Option 1: "The task had been finished by him, before he was left."

- Passive voice.

- Implies someone left him, altering the original meaning.

- Option 2: "The task was finished by him, before he left."

- Passive voice.

- Tenses shift slightly; doesn't maintain past perfect for clarity.

- Option 3: "The task had been finished by him, before he left."

- Passive voice.

- Maintains correct past perfect tense; matches original meaning.

- Option 4: "The task was finished by him before leaving."

- Passive voice.

- Loses specificity; "before leaving" could imply his or another's departure.