Which statement is NOT a best practice when using multiple ADs in OCI?

Keeping the primary and standby databases in the same Availability Domain (AD) is not a best practice when using multiple ADs in Oracle Cloud Infrastructure (OCI). The reason this approach is suboptimal is that it creates a single point of failure; if issues arise in that AD due to maintenance, outages, or other events, both the primary and standby databases could be affected simultaneously.

In contrast, utilizing separate ADs for primary, standby, and observer roles enhances high availability by ensuring that even if one AD experiences an issue, the others remain operational. Deploying load balancers across different ADs also supports application availability, as it allows for traffic management and redirection in the event of an AD failure. Additionally, leveraging fault domains when distributing instances helps isolate potential faults and increase the reliability of the infrastructure, but it operates within the context of a single AD.

Therefore, option A is recognized as not aligning with best practices, as the goal is to ensure redundancy and minimize the risk of simultaneous outages across your database resources.