Explanation:

- Option 1: "The song should be listened."

- This is incorrect. The verb "listen" needs a preposition.

- Option 2: "The song should be listened out."

- Incorrect usage. "Listen out" is not fitting here; it implies being alert for something, not passive listening.

- Option 3: "The song must be listened out."

- Also incorrect. "Must" implies obligation, and "listened out" isn't suitable here.

- Option 4: "The song should be listened to."

- Correct! The structure is proper. The preposition "to" is necessary after "listened."

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