Both young sleuths felt sure there was a link between the two.

Answer: Option D

Travellers : a person who is travelling or who often travels.
Boys : a male child or youth.
Convicts : declare (someone) to be guilty of a criminal offence by the verdict of a jury or the decision of a judge in a court of law.
Detectives : a person, especially a police officer, whose occupation is to investigate and solve crimes.
Sleuths : detectives.

Synonym of Sleuths is Detectives