Pointing to a boy, Sunita said "He is the son of my grandfather's only son". How is the boy related to Sunita?

Sunita's grandfather's only son would be Sunita's father. The boy is the son of Sunita's father, so he is Sunita's brother. Since "brother" is not among the options, the available data do not allow any listed exact match. Therefore, after checking all options, the correct choice is $\text{D}$.