The repetition of the work "singing" is associated with the enumeration of who is singing: for example the carpenter, the mason and the shoemaker.
Therefore, "singing" contributes to the feeling that everyone is singing: it provides a feeling of unity.
This singing also symbolizes hope for well-being: all those singing people have a job and a secure existence.
The correct answer is: The repetition emphasizes hope and a unified vision for America.