Final answer:
Class systems are open because they allow for movement between classes, driven by individual achievements and opportunities for social mobility.
Step-by-step explanation:
The factor that makes class systems open is a. They allow for movement between the classes. In contrast to closed systems like slavery and the caste system, open systems such as the class system are characterized by their flexibility and the opportunities they provide for individuals to achieve social mobility. People can move up or down the socioeconomic scale based on their individual achievements, educational attainment, and employment status, which facilitates interaction between different socio-economic layers.