The police tried to find out the rightful owner of the ornaments left in the train.

Answer: Option C

Rightful : having a legitimate right to property, position, or status.
Indefinite : lasting for an unknown or unstated length of time.
Mistaken : wrong in one's opinion or judgement.
Unlawful : not conforming to, permitted by, or recognized by law or rules.
Claimant : a person making a claim, especially in a lawsuit or for a state benefit.

Antonym of Rightful is Unlawful