Explanation:

- The original sentence is in the present perfect tense, "has returned," which is incorrect when specifying a past time like "a month ago."

- Option 1: No substitution required

- The original sentence is incorrect because it mixes present perfect tense with a specific past time reference.

- Option 2: would returned

- This option is grammatically incorrect. "Would returned" is not a valid tense in English. The correct form would be "would have returned," but this is still not suitable in this context.

- Option 3: returned

- The simple past tense "returned" is appropriate here. It correctly aligns with the time expression "a month ago."

- Correct Answer: "returned" accurately describes an action completed in the past.

- Option 4: was returned

- This option wrongly implies a passive action, which is inappropriate as the subject 'she' is active in the sentence.