The correct option is A b
Let’s examine the sentence: “The boat came abreast at us and signalled us to stop.” The sentence uses “abreast,” which means “next to each other, side by side, and facing in the same direction." So, the usage of “abreast” itself indicates that the boat came by the side of speakers and hence, does not require the use of preposition “at.” Therefore, “at” should be removed to correct the sentence. So, the error is in Part b of the sentence. Hence, option B is the correct answer.