Final answer:
The sentence from Mark Twain's excerpt that encapsulates his main argument about storytelling is that humorous stories are a high form of art that only artists can tell.
Step-by-step explanation:
The sentence that most clearly states the overall claim in the excerpt from Mark Twain's "How to Tell a Story" is: "The humorous story is strictly a work of art - high and delicate art - and only an artist can tell it; but no art is necessary in telling the comic and the witty story; anybody can do it." This sentence encapsulates Twain's main argument that the art of telling a humorous story is a unique and skilled practice, distinguishing it from other types of stories which he believes do not require the same level of artistry.