Which symbol listed below is not a standard Data Flow Diagram (DFD) symbol used to represent processes, external entities, data stores, or flows?

Introduction / Context:
Data Flow Diagrams (DFDs) model how data moves through a system. The notation uses a small set of shapes to keep diagrams simple and focused on processes, data stores, external entities, and flows.

Given Data / Assumptions:

• We refer to common Gane-Sarson or DeMarco/Yourdon styles.
• Typical symbols: external entity (square), process (circle or bubble/rounded), data store (open-ended rectangle), and arrows for data flows.
• The question asks for the item that is not standard.

Concept / Approach:
Both major notations agree on the fundamental shapes above. A triangle is not part of the core DFD symbol set; it can appear in other diagram types but not in standard DFD notation for the listed elements.

Step-by-Step Solution:
1) Map each option to a standard DFD symbol set.2) Confirm square = external entity.3) Confirm open rectangle = data store (open-ended).4) Confirm circle/bubble = process.5) Identify triangle as not used in standard DFDs.

Verification / Alternative check:
Reference guides and textbooks consistently show only the four shapes above for core DFD elements, with triangles absent.

Why Other Options Are Wrong:
A square: valid for external entities.An open rectangle: valid for data stores.A circle: valid for processes.A bubble: synonym for the process symbol.

Common Pitfalls:
Confusing DFDs with flowcharts or UML diagrams where additional shapes appear; mixing notations leads to miscommunication.

Final Answer:
A triangle