The correct answer is A. is singing.
The sentence “Listen, someone is singing a song” is in the present continuous tense. This tense is used to talk about actions that are happening at the moment of speaking. In this case, the speaker is hearing someone singing a song, so the action is happening at the moment of speaking.
The other options are not correct because they are not in the present continuous tense. Option B, sings, is in the simple present tense. This tense is used to talk about general truths or facts. In this case, the speaker is not talking about a general truth or fact, they are talking about something that is happening at the moment of speaking. Option C, singing, is the present participle of the verb to sing. This form is used to form the present continuous tense. However, in this case, the speaker is not using the present continuous tense, so the present participle is not correct. Option D, will, is the future tense. This tense is used to talk about actions that will happen in the future. In this case, the speaker is talking about something that is happening at the moment of speaking, so the future tense is not correct.