Capacity can be increased quickly without lengthy hardware purchases and High availability is simplified with built-in Multi-AZ deployments and read replicas are correct because Amazon RDS is a managed service that reduces hardware procurement delays and provides built in features for resilience and scaling.

Capacity can be increased quickly without lengthy hardware purchases is correct because RDS lets you change instance classes and adjust storage on demand or enable storage autoscaling so you do not wait for physical hardware purchases to grow capacity.

High availability is simplified with built-in Multi-AZ deployments and read replicas is correct because RDS can provision synchronous standby instances for automatic failover and manage asynchronous read replicas to improve read throughput and support disaster recovery.

You can manage the host operating system and install custom agents is incorrect because RDS does not expose operating system access or permit arbitrary software installation on the managed database hosts and you must use self managed servers when OS level control is required.

Cross-Region or cross-AZ database replication incurs no charges is incorrect because cross AZ and cross Region replication and data transfer typically generate storage and data transfer costs and some replication features have associated fees.

AWS Snowball is incorrect because Snowball is a physical device and service for offline bulk data transfer and it does not provide the managed database operational benefits described in this question.

Full AWS Practitioner Certification Question