Which clue helps readers identify tone in a text?

The correct option is b.

The tone of a text reflects the author's attitude toward the subject or audience. This attitude is primarily conveyed through the author's word choice (diction) and sentence rhythm (syntax and structure). For example, an author might use formal, academic language to convey a serious tone, or informal, humorous language for a lighthearted tone.

• a. The plot's resolution refers to how the story ends, which is a narrative element, not a direct indicator of tone.
• c. The number of characters is a structural aspect of the story, not related to the author's attitude.
• d. The setting's time period provides context but does not directly reveal the author's tone.

The final answer is $\boxed{\text{b. The author's word choice and sentence rhythm}}$.