Final answer:
In the context provided, chagrin refers to a feeling of distress or disappointment that arises from a negative event or realization. It indicates a profound emotional disturbance.
Step-by-step explanation:
The best synonym for chagrin, as used in paragraph 3, is distress or disappointment. The context describes a scenario of intense emotional upheaval. The character expresses a sensation of being overwhelmed and a sudden, dispiriting realization that their plans may have been compromised. These feelings encapsulate the essence of chagrin, which implies a deep emotional disturbance typically stemming from disappointment, embarrassment, or failure.