The correct answer to this open question is the following.
Unfortunately, you did not attach the excerpt, text, or the link to it, to know exactly what you mean by "sentence 5." Without this information is so difficult to help you.
However, trying to help you and by doing some deep research, we can say the following.
The revision of sentence 5 that creates the most effective thesis for this paper would be this one: "More interesting, however, is how this little dog earned the rank of U.S. Army sergeant and became a war hero."
This is the correct answer because, in sentence 5, Nick has not offered a strong controlling idea or thesis statement.
We are talking about the story of Stubby, a dog found in a military field in New Haven, Connecticut by a young man named Robert Conroy. He was a private in the military. The time, around 1917.