Final answer:
The most appropriate option is A. An event list, which outlines external events that can trigger system behavior and defines system boundaries.
Step-by-step explanation:
The option most appropriate to fill in the blank of the statement that identifies external events that could trigger behavior in a system is A. An event list. An event list includes all the external events that can impact the system and outlines how the system should respond to these events. It serves to define the boundary of the system's scope by detailing potential business events initiated by users or other sources, temporal events that are time-triggered, and signal events received from external hardware devices.