Final answer:
A public transportation network developer falls under the tertiary sector of economic activity.
Step-by-step explanation:
The type of economic activity that a public transportation network developer falls under is the tertiary sector. The tertiary sector is focused on providing services, such as transportation, healthcare, and child care. Public transportation network development involves the planning, designing, and managing of transportation systems for the public, which aligns with the service-oriented nature of the tertiary sector.