Final answer:
Twain uses the idiom 'like sardines' to compare the closely packed boats at the wharf to sardines.
Step-by-step explanation:
The idiom in the excerpt from 'Life on the Mississippi' by Mark Twain is:
'lay packed together like sardines'
This idiom is used to compare the boats to sardines, emphasizing how closely packed they are at the wharf. It creates a vivid image in the reader's mind of the crowded and cramped conditions.
The correct answer to the student's question is option 3) To compare two unlike things, boats and sardines, by using the word 'like'.