Seen this setup before, you'd use D. Multi-exit Discriminator (MED) helps your on-prem router know which Cloud Router should be primary for inbound BGP traffic. It's not about Local Preference since that's more for outbound path selection. Pretty sure it's D, but if someone disagrees let me know.

Actually, Multi-exit Discriminator (D) is the standard way to influence inbound traffic from the on-prem router to GCP. MED lets you signal your preferred path when there are multiple entry points, so setting a lower MED on the active router makes it primary. I think that's what Google Cloud expects here. Open for debate if someone disagrees.

It's actually D in this scenario because MED tells your on-prem router which Cloud Router path to prefer for inbound routes. If you set a lower MED on the active Cloud Router, that one gets picked for incoming traffic. Local Preference (C) would influence outbound from your own side, not which Google peer is active. I think that's how most exam guides explain it, but if anyone's got a real-world case where C worked, let me know!