Increase the DB instance class to a larger size and Expand the allocated storage for the DB instance are the correct ways to scale an Amazon RDS for PostgreSQL instance using built in RDS features without changing the application.

Increase the DB instance class to a larger size provides more CPU memory and network capacity which improves query throughput and concurrency for heavier workloads and it is a supported vertical scaling operation for RDS instances.

Expand the allocated storage for the DB instance increases available disk space and for many storage types it can raise IOPS and reduce storage related bottlenecks and RDS supports online storage scaling for most engines.

Use Amazon RDS Proxy to scale the database is incorrect because RDS Proxy pools and manages connections to improve resilience and efficiency but it does not add compute or storage capacity to the database engine.

Automatically change instance size with AWS Auto Scaling is incorrect because the AWS Auto Scaling service does not resize RDS instance classes and RDS instance class changes are handled by RDS specific operations or scheduling features.

Use EC2 Auto Scaling to add additional DB instances is incorrect because EC2 Auto Scaling manages EC2 instances and cannot create or scale managed RDS database instances and RDS does not rely on EC2 Auto Scaling groups for database scaling.

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