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.