Question

which statement primarily appeals to ethos to support the position that to improve their health, people should stand more and sit less?
after switching to a standing desk—like many americans, i used to spend most of my day sitting—i experienced subtle but meaningful health benefits: i improved my posture and found i had slightly more energy.
numerous studies have shown that simply standing more can improve the overall health of sedentary people.
by now, most people have heard the news that sitting for long periods of time is not just mildly unhealthy but can actually be deadly.

Explanation:

Ethos is an appeal to credibility or character. The first statement uses a personal experience ("After switching to a standing desk - like many Americans, I used to spend most of my day sitting - I experienced subtle but meaningful health benefits...") which builds the speaker's credibility as someone who has tried the change (standing more) and can speak from personal knowledge. The second statement ("Numerous studies have shown...") is an appeal to logos (logic, evidence from studies). The third statement ("By now, most people have heard...") is more of an appeal to pathos (emotion, using the idea of something being "deadly" to evoke fear) or just a general statement without a strong ethos appeal.

Answer:

The first statement ("After switching to a standing desk—like many Americans, I used to spend most of my day sitting—I experienced subtle but meaningful health benefits: I improved my posture and found I had slightly more energy.")