Final Answer:
In a Data Flow Diagram (DFD), the symbol of two parallel lines represents a Data Flow is 4) Data flow symbol
Step-by-step explanation:
This symbol is used to depict the movement of data from one process to another or from a process to a data store. The two parallel lines indicate the direction in which data is flowing in the system. It signifies the transfer of data from a source to a destination within the context of the diagram.
A Data Flow Diagram is a graphical representation of the flow of data within a system, and various symbols are employed to represent different elements. The symbol of two parallel lines specifically denotes the flow of data, making option 4, "Data flow symbol," the correct choice.