1. The problem asks: What is a relation that is reflexive, anti-symmetric, and transitive called? 2. Recall the definitions: - Reflexive: Every element is related to itself. - Anti-symmetric: If $aRb$ and $bRa$, then $a = b$. - Transitive: If $aRb$ and $bRc$, then $aRc$. 3. A relation with these properties is called a **partially ordered set (poset)**. 4. Therefore, the correct answer is **C) Poset**.