The second thesis statement, "Because junk food is bad for your health, the size of sodas offered at fast-food restaurants should be regulated by the federal government," is the one that can be argued and proven with evidence in an argumentative essay.
This statement presents a clear position that can be supported with various types of evidence, such as health studies linking junk food and soda consumption to negative health outcomes, statistics on obesity and related diseases, as well as arguments regarding government intervention in public health.
In contrast, the first statement, "Junk food is bad for your health," is more of an assertion and serves as a premise rather than a stance that can be argued. While it can be supported with evidence, it doesn't present a specific argument or call to action like the second statement does.