Explanation:

- Robert K. Merton is the thinker associated with the concept of reference group behavior, which examines how individuals evaluate themselves by comparing with others.

- Option 4: Merton

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- Karl Marx focused on conflict theory and class struggles, not specifically on reference groups.

- Talcott Parsons is known for his work on structural-functionalism rather than reference groups.

- Max Weber is known for his theories on bureaucracy and social stratification, not directly on reference groups.