Final answer:
The young waterfowl's motivation to try to flee from the model of a bird with a short neck can best be explained by the instinct theory.
Step-by-step explanation:
The young waterfowl's motivation to try to flee from the model of a bird with a short neck like a hawk, who are a threat to them, can best be explained by the instinct theory. Instinct theory suggests that certain behaviors are innate and instinctual, meaning they are genetically programmed and do not require learning or experience. In this case, the waterfowl's instinctual response to a potential predator with a short neck triggers their motivation to try to escape.