Answer:
The sentence that best fits the definition of a thesis statement is c. The United States party system has helped the country maintain a balance between strong national government and the rights of its states and citizens.
Step-by-step explanation:
A thesis statement is a sentence that expresses the main idea of the essay or research paper and, at the same time, shows the purpose of this paper. Sentence C is the correct answer because it establishea the direction of the entire research and narrows the topic to a specific focus of investigation.