Final answer:
In the context of the Great Emu War passage, 'bird' is the synonym for avian (likely intended to be 'avian'), since avian pertains to birds. The other options relate to flight but do not refer specifically to birds.
Step-by-step explanation:
The word avain likely refers to 'avian' which is related to birds. Therefore, the synonym for avian in the context of the Great Emu War passage would be bird. The other options given, such as flying, aerial, and plane, are all related to flight but do not specifically refer to 'bird' which is the direct synonym for avian. Among the provided choices, bird emphasizes the creature itself rather than the action of flying or the means by which flying is achieved (plane).
While the term 'avian' is generally associated with birds, in the context of the passage, it specifically excludes other concepts like flying ability, as not all avian creatures are capable of flight, as demonstrated by the emu and other flightless birds mentioned. Hence, the term 'avian' characterizes birds by their group or class rather than by the act of flying.