Final answer:
The guideline for effective informative speaking discussed in the textbook is providing a clear thesis statement. Therefore, the correct option is D.
Step-by-step explanation:
The correct answer is D. Providing a clear thesis statement. A clear thesis statement is an important guideline for effective informative speaking because it helps the audience understand the main idea and purpose of the speech. It provides a roadmap for the speaker and allows them to organize their supporting points and evidence in a coherent manner.