Select the option that can be used as a one-word substitute for the given group of words.

A letter, poem etc. whose author is unknown.

Anonymous

The one-word substitute for "a letter, poem etc. whose author is unknown" is anonymous.

• Autonomous means self-governing or independent.
• Anomalous means unusual or irregular.
• Autocrat means a ruler with absolute power.

Anonymous means of unknown authorship or origin. It is the most appropriate word for the given group of words because it specifically refers to something that has no known author.