C. It helps the reader visualize the setting.
The passage is describing what the narrator sees. It talks about how the fish looks in the sunlight. It also describes the fish that are jumping out of the water and how the surface of the water looks as the fish churn it up. We don't actually know the feelings of the man, so we also don't know his mood. There aren't any descriptors about the setting that give a clear indication of the story's tone either.