I believe the answer is: A. But we had not traveled fifty miles down the stream before we found the water gradually becoming brackish anddiscolored from the saltand alkali in the soil,
In first person Point of view, the narrator the main character in the story and the story is depicted according to the perspective of the main character.
First person point of view writing could be easily identified by the usage of "I" and "We" for single and plural pronouns in the story.