Final answer:
The United States has an open stratification system known as a class system, which allows for social mobility based on achievement. The correct option is a. open.
Step-by-step explanation:
The United States operates under a class system, which is considered to be an open stratification system. This means that the social position of individuals and groups can change based on achievement. Factors such as wealth, income, education, and occupation play a significant role in determining one's social class, with movement and interaction between classes being possible.
For instance, someone can move from being in the lower class to the middle class through education and career advancement, thus achieving a higher socioeconomic status.
Unlike closed systems like caste or slavery, where social mobility is restricted and social position is ascribed based on birth, the class system in the U.S. allows for the possibility of social mobility.