In a standard Entity–Relationship Diagram (ERD), which of the following shapes is <em>not</em> part of the core notation set?

Introduction / Context:
ERD notation relies on a small, consistent set of shapes so that designers and readers can communicate data models without ambiguity. This question asks you to spot the symbol that is not standard in classic ERD notation.

Given Data / Assumptions:

• Classic Chen-style ER notation uses rectangles, ovals, and diamonds.
• Extended or alternative notations may add crow’s feet or special markers, but they do not change the basic symbol set.

Concept / Approach:
Standard ERD notation uses a rectangle for entities, an oval for attributes, and a diamond for relationships. A generic standalone circle is not part of this core set in standard ERD notation.

Step-by-Step Solution:

Verification / Alternative check:
Textbook ERD references and modeling tools confirm that circle is not a primary symbol in ERD; attribute variants use double ovals rather than circles.

Why Other Options Are Wrong:

• Rectangle: standard for entities.
• Oval: standard for attributes.
• Diamond: standard for relationships.

Common Pitfalls:
Assuming a circle is just a stylized oval; in diagramming conventions, the oval is the attribute symbol and is usually drawn elongated, not as a perfect circle.

Final Answer:
Circle