Final answer:
The sentence 'Rachel McCrystal, Executive Director of Woodstock Farm Sanctuary, notes that factory farming is problematic for the baby animals it harms, the humans who work there, and the future of our planet.' is the best example of using a signal phrase to cite a source.
Step-by-step explanation:
The sentence that best uses a signal phrase to internally cite a source is: "Rachel McCrystal, Executive Director of Woodstock Farm Sanctuary, notes that factory farming is problematic for the baby animals it harms, the humans who work there, and the future of our planet." This sentence effectively uses a signal phrase by introducing the source and its credibility (Executive Director of Woodstock Farm Sanctuary), then presents the claimed ideas with a verb (notes) that reflects the author's stance on the issue. This provides enough context for readers to understand the quotation and implies the authenticity of the information.