One of the most important characteristics of systems of stratification is their degree of flexibility. in a closed system of stratification
a) people's positions are set by achieved statuses
b) there is little or no social mobility
c) the boundaries between levels in the hierarchies of social stratification are relatively fluid
d) many opportunities exist for people who are willing to work hard

The correct answer is B. There is little or no social mobility

Explanation:

In societies, a system of stratification refers to a hierarchy that organizes people in different categories or social classes and each of these has a different level of power, features, and roles in society. Additionally, these systems can be classified as open in the case their boundaries are flexible or fluid and people can achieve different status or ascend into the hierarchy or as closed in the case of systems with no o little mobility, which implies people remain in the same social class or category they were born and the boundaries between different classes are rigid. Considering this, it can be concluded in a closed system of stratification there is little or no social mobility.