I would say Microsoft Power Automate flows

Normally I'd go with what everyone else said and say D for Power Automate Flows, but you all missed on key point in the question. The other system is referred to as a "proprietary system", meaning there would not be a connector for Power Automate. So My answer is B: Custom Connector.

Vote D

A lot of information is missing here... System is proprietary... Ok, but does it mean it is Web API? If so, custom connector will help partially. Custom connector for itself means nothing. You must use it inside of an application or Power Automate flow. Maybe I will choose option D - Power Automate Flow, but how to connect to the "proprietary" system? Which connector I have to use? In practice I will leverage custom connector inside of a Power Automate flow. But here... I don't know... Combination of both option represents the complete solution.

I am quite sure is Power Automate. In the question says that this will relay on Employees, so for me this is a citizen developer case, and they should use Power Automate. The key part of the question is here: "Employees are not able to provide a definite schema for their data". This falls under the Citizen Developer Role to me :) https://www.microsoft.com/insidetrack/blog/unleashing-the-citizen-developer-in-all-of-us-with-the-microsoft-power-platform/

Option C, Data gateways, are used to provide a secure connection between your on-premises data sources and cloud services like Power BI, Power Apps, Power Automate, and Azure Logic Apps. They are not typically used for configuring automatic updates between two systems, especially when one of them is a proprietary system with an undefined schema. On the other hand, Microsoft Power Automate flows (Option D) can be used to create automated workflows between different applications and services, which fits the requirement of configuring automatic updates between the proprietary system and Microsoft Dynamics 365 Project Operations. It also allows employees to configure these workflows themselves, which is another requirement in the scenario. Therefore, while Data gateways (Option C) are useful in certain scenarios, they are not the best fit for this particular use case. Microsoft Power Automate flows (Option D) would be a more suitable choice

The system is proprietary

Microsoft Power Automate flows

Power automate flow

For a solution that allows employees to configure their own automatic updates from a proprietary system to Microsoft Dynamics 365 Project Operations, the most suitable component would be: D. Microsoft Power Automate flows Power Automate flows can automate the process of updating data in Dynamics 365 from various sources, including proprietary systems. If there is no predefined schema, flows can be designed to handle conditional logic and data mapping, allowing for a flexible update process that employees can customize as needed.

D. Microsoft Power Automate flows