Ensure that Multi-AZ is enabled for Amazon Relational Database Service (Amazon RDS) Instances

Multi-availability zone architecture can provide redundancy for a more resilient database infrastructure. RDS instances can be configured in clusters that cross availability zones for MySQL and PostgreSQL database engines. For more information, see the AWS documentation.
References:
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/create-multi-az-db-cluster.html

In AWS Console -

1. Sign in to the AWS Console and go to the AWS RDS Console.
2. In the RDS Dashboard, click on Databases.
3. Select Modify to modify the instance of your choice.
4. Modify Multi-Az to ensure database is available in multiple availability zones.

In Terraform -

1. In the aws_db_instance resource, set the multi_az field to true.

References:
https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance#multi_az