The correct answer is: D. is being sung
The sentence “Listen, a nice song is being sung” is in the present continuous tense. This tense is used to talk about things that are happening right now. In this case, the speaker is saying that they can hear a nice song being sung.
The other options are incorrect because they are not in the present continuous tense. Option A, “is singing,” is in the present simple tense. This tense is used to talk about things that happen regularly or repeatedly. Option B, “has sung,” is in the past participle form. This form is used to talk about things that have happened in the past. Option C, “was being sung,” is in the past progressive tense. This tense is used to talk about things that were happening in the past.
In conclusion, the correct answer is: D. is being sung.