The correct passive form of the given sentence is:

☀ By whom is such a sweet song being sung?

 Let's break down the options:

1. By whom is such a sweet song sung? (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence.

2. By whom is such a sweet song being sung? (Incorrect) This option uses the present continuous tense "is being sung," which implies an ongoing action at the present moment. The original sentence doesn't indicate an ongoing action but simply asks about the performer of the action, as shown in option 1.

3. By whom is such a sweet song sang? (Incorrect) This option uses the incorrect past participle "sang." The correct past participle of "sing" in this context is "sung," as shown in option 1.

4. By whom has such a sweet song sung? (Incorrect) This option uses the present perfect tense "has sung," which implies that the singing action has been completed in the past with present relevance. The original sentence is in the present tense, not the present perfect tense, so option 1 better reflects the intended meaning.