Which type of mobility is evident when higher-class couples have comparatively fewer children than lower class couples, thus making room for bright and hardworking offspring of lower-class parents to move up?

Class mobility is a type of social mobility when representatives of a lower-class move to the higher class. It can occur as a result of changes in both classes.

In the given example it occurs both because of the fact that families in the higher class have fewer children and because some offsprings from a lower-class families are hardworking. The tendency that higher-class couples tend to have fewer children leads to the more available spaces in the entry-level labor force, which in turn creates more real opportunities for children from lower-class families. This in is itself becomes an extra motivator to work harder.