Explanation:

- Option 1: Spy

- A spy is someone who gathers secret information, often for intelligence purposes. This person is typically employed by governments or organizations.

- Option 2: Eavesdropper

- An eavesdropper is a person who secretly listens to others' private conversations without their knowledge. They do this out of curiosity or to gather information, but not usually as part of a professional role.

- Option 3: Secret agent

- A secret agent is a person who works covertly, often engaging in espionage or undercover activities. They gather classified information and may have a formal role or assignment.

- Option 4: Infiltrator

- An infiltrator is someone who gains access to a place or group covertly, often to gather information or sabotage from within. They are part of covert operations or espionage activities.