Final answer:
Laura feels ill when the gentleman caller arrives because she is overwhelmed by his presence.
Step-by-step explanation:
Laura feels ill when the gentleman caller arrives because she is physically and emotionally overwhelmed by his presence. This can be seen in the extract when Laura is described as being 'white and haggard' and the gentleman caller is 'imperturbable'. The contrast between Laura's state and the caller's calmness suggests that Laura's physical condition is a result of her nervousness and anxiety.