The correct answer is: Secret.
A secret is something that is kept hidden or private, and that should not be told to anyone else. It is often something that is embarrassing or shameful, or that could cause harm to someone if it were to be revealed.
Hidden is something that is not easily seen or found. It may be hidden because it is being kept secret, or because it is not important enough to be seen.
Private is something that belongs to a particular person or group of people, and that is not shared with others. It may be private because it is personal or confidential, or because it is not something that the person or group wants others to know.
Trusted is someone or something that is believed to be honest and reliable. A trusted person is someone who can be relied on to keep secrets and to do what they say they will do. A trusted thing is something that is believed to be accurate and reliable.
In this case, the correct answer is “secret” because it is the closest synonym to “confidential.” Confidential information is information that is not to be shared with anyone else, and that should be kept secret.