Final answer:
In audience setup within Campaign Manager, Location is often the mandatory targeting field needed for defining the audience to ensure proper ad distribution and adherence to advertising regulations.
Step-by-step explanation:
The question deals with the audience setup within Campaign Manager, which is a part of digital advertising technology. Among the options given - Age, Gender, Location, and Interests - the mandatory targeting field required for audience setup is typically Location. While Age, Gender, and Interests might be optional or based on the specific campaign goals and available data, Location is often a fundamental targeting criterion that must be defined for proper ad distribution and compliance with legal advertising regulations.