A company uses Amazon RDS and needs to ensure the database is highly available. Which RDS feature should be used?

Multi-AZ deployment is the correct choice for high availability in Amazon RDS. It automatically provisions and maintains a synchronous standby replica of your database in a different Availability Zone within the same AWS Region. In the event of an infrastructure failure (e.g., instance failure, storage failure, or AZ outage) affecting the primary database, RDS automatically fails over to the standby replica, minimizing downtime without manual intervention. Read replicas improve read throughput and can be used for disaster recovery but are not synchronous and do not provide automatic failover for the primary instance. Blue/green deployment is for updating databases with minimal downtime, not for high availability of a single instance. Reserved Instances are a billing discount option and have no impact on database availability.