IN: Which person is they?
People raise dogs because they are afraid of thieves.
OUT: People

IN: Which person is he?
The Centurion lent Doctor Who his ear since he had pledged his loyalty.
OUT: The Centurion

IN: Which person is they?
The collections agency often goes after delinquent accounts, but they sometimes make mistakes.
OUT: The collections agency

IN: Which person is they?
The NSA sent their agents in the field to retrieve intelligence, but they were not able to gather the intelligence for them.
OUT:
their agents