The correct answer is: D. is being sung.
The sentence “Be quiet. A new song is being sung.” is in the present progressive tense. This tense is used to describe actions that are happening at the moment of speaking. In this case, the speaker is telling the listener to be quiet because a new song is being sung.
The other options are incorrect because they do not accurately describe the action of the sentence. Option A, “is sung,” is in the simple past tense. This tense is used to describe actions that happened at a specific time in the past. Option B, “is singing,” is in the present tense. This tense is used to describe actions that are happening regularly or repeatedly. Option C, “was sung,” is in the simple past perfect tense. This tense is used to describe actions that happened before a specific time in the past.
In conclusion, the correct answer is: D. is being sung.