You're right to question this. While installing Defender for Endpoint on the macOS devices (option C) is indeed necessary to enable antivirus protection, the question specifically asks for the action that ensures you can apply antivirus policies with minimal administrative effort.
Installing Defender for Endpoint is just the initial step. However, creating a configuration profile (option D) allows you to define and apply antivirus policies centrally to multiple devices at once, which aligns with the requirement to minimize administrative effort. Therefore, option D is the more appropriate choice for achieving the stated objective.