What is the purpose of a story's tone?

To evoke an emotional response from the reader

to reveal the author's attitude about the characters and events

to foreshadow what the author knows but the characters do not

to create a visual picture of the setting for the reader

All of the above are purposes of a story's tone. The tone of a story sets the overall atmosphere and emotional experience for the reader. It can evoke specific emotions, reveal the author's attitude towards the characters and events, foreshadow future events, and help create a vivid picture of the setting in the reader's mind.