Explanation:

When designing a Microsoft Sentinel workspace, cost optimization and data retention management are two key considerations. Microsoft Sentinel stores data in an Azure Log Analytics workspace, and pricing for data ingestion and retention is managed through Log Analytics settings.
* Minimize costs for daily ingested data:Microsoft's documentation on Log Analytics pricing models states that you can choose between Pay-As-You-Go (PAYG) and Commitment Tiers. The Commitment Tier model allows you to commit to a fixed amount of daily ingestion (for example, 20 GB/day in this case) at a lower per-GB cost compared to PAYG pricing. If your ingestion volume is predictable (as in this scenario-20 GB per day), this model provides significant cost savings without the administrative overhead of managing caps or throttling.Therefore, to minimize ingestion cost, the correct choice is "Use a commitment tier."
* Maximize the data retention period without incurring extra costs:By default, Microsoft Sentinel (via Log Analytics) provides 90 days of data retention at no additional charge. Extending retention beyond 90 days incurs additional storage charges. According to Microsoft's official guidance, "Log Analytics retains data for 90 days at no cost; data kept beyond that period is billed at the retention rate." Therefore, to maximize the free retention period while avoiding extra cost, the correct configuration is
"Set retention to 90 days."
Summary:
* Minimize costs for daily ingested data # Use a commitment tier
* Maximize retention without extra costs # Set retention to 90 days
This configuration ensures both cost efficiency and maximum free data availability, aligning with Microsoft Security Operations (SecOps) and Sentinel best practices.