You are defining the project connection for the Mediated Connectivity deployment.

From the “Creating a Connection for Mediated Connectivity Integration” module on learning.sap.com, the System ID value becomes available in the System ID dropdown menu when system information has been added in the Resources (also called "Basic Data – Trading Partner System Information") section of the Spend & Network portal. SAP Learning

Among the provided options, the one that matches this description is:

A.When the System ID is made available in the Resources section of the Spend & Network portal

So the correct answer is A.