My father keeps all his . . . . . . . papers in a lock and key.

The correct answer is D. confidential.

The other options are not correct because:

• Required: This does not make sense in the context of the sentence.
• Necessary: This does not make sense in the context of the sentence.
• Useful: This does not make sense in the context of the sentence.
• Enclosed: This does not make sense in the context of the sentence.

Confidential papers are papers that contain private or sensitive information that should not be shared with others. They are often kept in a safe place, such as a lock and key, to protect their confidentiality.