Final answer:
Social stratification is the ranking of individuals in a hierarchy of inequality. The meritocracy system hypothetically permits the greatest degree of social mobility, unlike the closed nature of a caste system. Option d.
Step-by-step explanation:
The concept that is defined as the systematic ranking of groups of people in a hierarchy of inequality is social stratification.
Social stratification is a socio-economic system that divides society's members into categories ranking from different levels of social standing based on factors like wealth, income, education, family background, and power. Social mobility refers to the ability to change positions within this system.
Regarding the system that allows for the most social mobility, meritocracy is the hypothetical ideal system where social standing is based solely on personal effort and merit.
In contrast, a caste system represents a closed stratification system with ascribed status that typically allows for little or no social mobility, while class systems tend to be more open and allow for greater movement depending on individual achievements.
So Option d is the correct answer,