The answer is letter D.
If the poet replaced "belly" with "stomach", the tone would become more formal, since the word "belly" is classified as being of informal nature even in dictionaries. Therefore, if Jimmy Santiago had used the word "stomach", he'd have sounded more formal and less close to the person to whom the poem was directed when written. It would have been as if he were giving some sort of medical advice, and the degree of intimacy and friendliness suggested in "to warm your belly in winter", would have been much lower.