Final answer:
The DFD symbol that is usually portrayed with a square is the c)external entity, which indicates actors outside the system that interact with it.
Step-by-step explanation:
In the context of a data flow diagram (DFD), the DFDsymbol that is usually portrayed with a square is the option c)external entity.
An external entity, which can also be referred to as a source or destination, represents external agents or actors that interact with the system being diagrammed. This symbol is critical as it helps to delineate the boundaries of the system and shows how external factors or agents can affect or be affected by the system's data processes.