Which person did he see?
1. He saw the person in the room.
2. He saw a person in the room.
[Which answer is correct? Are both okay? Is there any difference between them? Does #1 mean that there is only one person in the room?]
"Which" indicates that he is looking for only one person, a specific person. In the answers, you need some specific wording to indicate that he saw the person he intended to find.
He saw his brother in the room.
He saw that tall man in the room.
He saw the doctor in the room.
etc.