Since this is a poem and you have a comma after river bank, the answer would be Halting because it is starting a new line. River should not be capitalized in this sentence because it is not a proper noun. It is just a common noun.
We need to be able to read the story before we answer.
Overall, Thoreau wanted to prove that not everyone will easily flow with the rules of society. As a result, he calls the people who go different paths than the ones of society "different drummers."
In conclusion, your final answer should be A.
