The answer to your question would be that the answer that summarizes the meaning of the sentence correctly is the following one: An understatement describing a bad argument using analogy.
An understatement is the presentation of something as being smaller or less good or important than it really is. By saying that Mount Everest, an enormous mountain, and the Mississippi River, a huge river, are just a hill and a trickle of water just like the disagreement, you are stating that what happenned was more than a minor disagreement.