Given workloads (20 TB legal log archive; 500 GB VM boot/data volumes and templates; 500 GB image thumbnails; 200 GB customer session state persisted for days), what is the most cost-effective storage allocation?

The correct option is the most cost-effective. Customer session state requiring persistence for days is best handled by a NoSQL database like Cloud Datastore (now Firestore in Datastore mode) for scalability and cost, with memcache for speed. Logs (20 TB) and thumbnails (500 GB) are ideal for Cloud Storage with lifecycle management to move older data to colder, cheaper tiers. VM boot/data volumes (500 GB) are also suitable for Cloud Storage, which can serve as a repository for disk images and snapshots. Incorrect options: Local SSDs are expensive and ephemeral, unsuitable for persistent session state or VM boot volumes. Cloud SQL is a relational database, less cost-effective for simple session state than Datastore. Persistent Disk SSDs are also more expensive than Datastore for this use case.