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

A person who listens to someone’s private conversation without them knowing

Eavesdropper

The word that can be used as a one-word substitute for "a person who listens to someone's private conversation without them knowing" is eavesdropper.

• Spy is a person who secretly gathers information about an enemy or competitor.
• Eavesdropper is a person who listens to someone's private conversation without them knowing.
• Secret agent is a spy who works for a government.
• Infiltrator is a person who secretly enters a group or organization in order to gather information or cause harm.

Eavesdropper is the most appropriate word for the given group of words because it specifically refers to someone who listens to someone else's conversation without their knowledge or consent. It is derived from the Middle English word eavesdroppere, meaning "to listen secretly to a private conversation."