The best statement that explains how hints in the passage allow you to infer that the creature in the river is actually the monster walking along it is:
We know that the creature in the river is the monster because the text states that it “continues to follow him” and then disappears when "he moved away" from the river.
This option directly connects the creature's behavior of following the monster and disappearing when the monster moves away from the river, indicating that they are the same entity.