Sustained-use discounts are automatically applied by Google Cloud when Compute Engine instances are run for a significant portion of the month. These discounts do not require any prior commitment or action from the customer; they are automatically calculated based on usage.
Option B: Nothing. Sustained use discounts are automatically applied is correct because Google Cloud automatically grants these discounts based on usage patterns.
References:
* Google Cloud Pricing: Sustained Use Discounts Explanation
* Google Cloud Compute Engine Pricing Guide

Turn on committed use discount sharing, and create a commitment for the combined usage Sharing your committed use discounts across all your projects reduces the overhead of managing discounts on a per-project basis, and maximizes your savings by pooling all your discounts across your projects' resource usage. If you have multiple projects that share the same Cloud Billing account, you can enable committed use discount sharing so all of your projects within that Cloud Billing account share all of your committed use discount contracts. Your sustained use discounts are also pooled at the same time. That is, sustained use discounts are calculated using the total resources across these projects, rather than just the resources within a single project.
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Reference link- https://cloud.google.com/compute/docs/instances/signing-up-committed-use- discounts#sharing_committed_use_discounts_across_projects