Final answer:
Birbal Shank's unassuming nature is shown when he responds with amusement and a slight smile after being by a young woman with a bow.
Step-by-step explanation:
In the story, the instance that shows Birbal Shank's unassuming nature is when he was by a young woman with the sharp end of her bow. Instead of reacting with anger or aggression, Shank responded with amusement and a slight smile. This demonstrates his humble and unaffected demeanor, as he did not take offense or seek revenge, but simply acknowledged the situation.