The standard Oracle Autonomous AI Database Serverless resource-allocation interface permits the administrator to configure the base compute allocation up to 512 ECPUs using the ECPU slider. This value represents the configured base ECPU count for a single database instance rather than the potential temporary compute consumption obtainable through compute auto scaling.
Oracle distinguishes configured ECPUs from auto-scaled usage. For eligible Autonomous AI Database configurations, compute auto scaling can allow the database to consume additional processing capacity above the configured base according to workload demand. For example, a database configured with 512 ECPUs may be capable of consuming substantially more effective processing capacity when auto scaling is enabled, subject to the service and licensing model.
The 1024-ECPU option therefore does not represent the normal single-instance slider ceiling addressed by this question. The 256- and 128-ECPU values are valid intermediate configurations but are below the maximum.
Consequently, A. 512 ECPUs is correct.
Study Guide reference: Provision and operate Oracle AI Database@Azure Resources - Autonomous AI Database Serverless provisioning, ECPU configuration and compute scaling.