Answer:
C. The structure is by event; the style is informal.
Explanation:
The structure of the excerpt is organized by events - detailing the activities the writer and their siblings engaged in during their vacation in London. The style, however, is informal, characterized by colloquial language, casual tone, and use of idiomatic expressions like "hit the hay" and "hoofed it around town." Therefore, the correct answer is:
C. The structure is by event; the style is informal.