Final answer:
Beef barley soup is the least likely to be considered strictly broth-based compared to the other options, as it is often thickened with barley.
Step-by-step explanation:
The question asks about which option is NOT a broth-based soup. In the list provided, mixed vegetable, chicken gumbo, and chicken noodle are broth-based soups typically made with a clear stock or broth as a base. However, beef barley soup, while it can be made with broth, is often thickened with the starch from barley, potentially making it more of a stew than a typical broth-based soup.
Therefore, although it can sometimes be made with a broth base, based on the potential for a thicker consistency, beef barley would be the least likely to be considered strictly broth-based among the choices given.