The most accurate conclusion to draw about the level of economic development in a country where most workers are in the tertiary and quaternary sectors is:
Countries in which most workers are in these sectors typically have a high level of economic development.
This is because the tertiary sector (services) and quaternary sector (knowledge-based services) are generally associated with advanced economies that are more developed, while primary (agriculture) and secondary (manufacturing) sectors are more common in less developed economies.