Final answer:
The sentence most likely to build an author's ethos appeal is: 'As the head lifeguard on Kidd Lake, I'm responsible for safe swimming and boating.'
Step-by-step explanation:
The sentence most likely to build an author's ethos appeal is: As the head lifeguard on Kidd Lake, I'm responsible for safe swimming and boating. This sentence establishes the author's credibility and expertise by highlighting their position as the head lifeguard on the lake. It shows that they have knowledge and experience in ensuring safety during swimming and boating activities.