Let's analyze the relationship step by step.
Ramya says: "He is the son of the only son of my Grandfather."
- The "only son of my Grandfather" refers to Ramya's father, because her grandfather has only one son, which is Ramya's father.
- The boy in question is "the son of Ramya's father," which means the boy is Ramya's father's son.
- Therefore, the boy must be Ramya's brother.
So, the relationship of the boy to Ramya is her brother.
Hence, the correct answer is option (B) Brother.