What kind of family is made solely of unmarried adults with no children?

The most suitable choice for a family solely made up of unmarried adults without children is the childless family. A childless family refers specifically to adults who choose not to have children or are unable to have them, and this category can include unmarried individuals who live together or share a household.

The other options do not fit this description as accurately. For instance, a foster family typically involves adults who provide care for children who are not their biological offspring, making it distinct from the definition of a childless family. A communal family generally consists of a group of individuals living together who may or may not be related and often share resources and responsibilities, which can include various family structures beyond just unmarried adults. Lastly, a nuclear family consists of two parents and their children, which again does not align with the criteria of having no children and only unmarried adults. Thus, "childless family" is the most appropriate choice in this context.