Thesis refers to the statement which put forwards own belief and what is intended to prove. A good thesis statement helps to focus on important information.
Hence, a sentence which is the thesis in this paragraph is A)
Because popcorn is relatively high in nutritional value, is fairly inexpensive, and lends itself to a variety of cooking methods, it is one of the most popular American snack foods.
The above thesis statement compared to other statements focuses on the main point to be conveyed. As the above paragraph talks about the healthy snack, popcorn, it primarily focuses on its consumption level and how it is a healthy snack at a cheaper rate.