What would happen to an RDS (Relational Database Service) Multi-Availability Zone deployment if the primary DB instance fails?

Answer options:

A.The IP address of the primary DB instance is switched to the standby DB instance.
B.The primary RDS (Relational Database Service) DB instance reboots and remains as primary.
C.A new DB instance is created in the standby availability zone.
D.The canonical name record (CNAME) is re-pointed from the primary to the secondary instance (standby).

Answer – D
Option A is incorrect because the IP address of the primary and standby instances remain the same and are not changed.
Option B is incorrect because the CNAME record of the primary DB instance changes to the standby instance.
Option C is incorrect because there is no new instance created in the standby AZ.
Option D is CORRECT because the CNAME of the primary DB instance changes to the standby instance so that there is no impact on the application setting or any reference to the primary instance.
More information on Amazon RDS Multi-AZ deployment:
Amazon RDS Multi-AZ deployments provide enhanced availability and durability for Database (DB) Instances, making them a natural fit for production database workloads. When you provision a Multi-AZ DB Instance, Amazon RDS automatically creates a primary DB Instance and synchronously replicates the data to a standby instance in a different Availability Zone (AZ). Each AZ runs on its own physically distinct, independent infrastructure, and is engineered to be highly reliable. In case of an infrastructure failure (for example, instance hardware failure, storage failure, or network disruption), Amazon RDS performs an automatic failover to the standby so that you can resume database operations as soon as the failover is complete. 
And as per the AWS documentation, the CNAME is changed to the standby DB when the primary one fails.
https://aws.amazon.com/rds/faqs/
For more information on Multi-AZ RDS, please visit the link-
https://aws.amazon.com/rds/details/multi-az/