Final answer:
Mark Twain uses irony in the excerpt to contrast the boys’ unexpected feeling of contentment over a boat explosion with the typical concern one might expect, highlighting this through the returned character being viewed as a hero against the expected criticism of Providence's partiality.
Step-by-step explanation:
Mark Twain uses irony in the excerpt from Life on the Mississippi in a way that contrasts the boys' expectations with reality. The options provided suggest different uses of irony, but the correct answer, according to the context provided, is the one that highlights the disparity between the boys' feelings and what one might expect in such a situation. Specifically, Twain utilizes irony when he notes how the boys felt a 'tranquil contentment' over the boat explosion, which is unexpected, as opposed to concern for the character's wellbeing. The irony heightens when the character, after surviving the explosion, comes back as a 'shining hero,' thus critiquing the partiality of Providence. This creates a contrast not because of the cub engineer's attitude or the boys' view of his injuries, but because their actual emotional reaction of satisfaction was the opposite of the expected concern over such a grave event.