Which of these rhetorical devices does thoreau most clearly use here?

The answer is:

Rhetorical question

Explanation:

Rhetorical questions don't require an answer and are asked merely for effect and influencing the decission or outcome.

B. Rhetorical question

Explanation:

A rhetorical question is an "apparent question" as its purpose is not to get an answer (in fact, it does not require one) but to make a statement, and lay emphasis on some point. In the excerpt, that is what the speaker aims to do. He or she begins by stating a point: "unjust laws exist", and provides a question with several options (as if to make the reader think), and ends the question with a final option "or shall we transgress them at once?" The author is not really asking, his purpose is to lay emphasis in the point that laws are unjust and therefore, we should do something about, we should transgress them at once, and fight to make them just.

ok

explanation:

those who allied with france gained land and power.