A girl introduced a boy as the son of the daughter of the father of her uncle. The boy is related to…

Tracing the family relationship step by step: 1. 'Father of her uncle' = the girl's grandfather (paternal or maternal). 2. 'Daughter of the father of her uncle' = the daughter of the girl's grandfather, who is the girl's mother (or aunt). 3. 'Son of the daughter of the father of her uncle' = the son of the girl's mother, who is the girl's Brother (or cousin if the daughter is an aunt; since 'Cousin' is not among the four options—Brother, Uncle, Nephew, Son—Brother in is the unique valid relationship).