The correct answer is D. has been singing.
The sentence is in the present perfect continuous tense, which is used to talk about an action that started in the past and continues up to the present. In this case, the action is “singing professionally.” The mother is telling the speaker that Tony has been singing professionally since he was four years old. This means that he started singing professionally when he was four and has continued to do so ever since.
The other options are not correct because they do not express the idea that Tony has been singing professionally for a long time. Option A, has sung, is in the simple past tense, which is used to talk about an action that happened at a specific time in the past. Option B, was singing, is in the past continuous tense, which is used to talk about an action that was happening at a specific time in the past. Option C, is singing, is in the present tense, which is used to talk about an action that is happening now. Option E, sings, is in the simple present tense, which is used to talk about an action that happens regularly or repeatedly.
In conclusion, the correct answer is D. has been singing.