Final answer:
The key milestone for the object-relational model was the creation of a single structure combining data and relationships, thus enhancing database technology.
Step-by-step explanation:
The key milestone for the object-relational model created in the mid-1980s was A. the creation of a single structure that contained both data and its relationships. This advancement allowed for the incorporation of complex data types and the relationships between data to be integrated into one system.
This was a significant development in database technology as it extended the relational database model to support more complex applications and types of data.