How should a solutions architect resolve this issue with the LEAST amount of application changes?

Explanations:

Adding a secondary DB instance using Multi-AZ primarily enhances availability and failover capabilities but does not help with read scalability or offloading read queries from the primary instance, which is necessary to address the increased query load from the custom reporting application.

Setting up a read replica in conjunction with Multi-AZ provides both read scalability and high availability. The read replica can offload read queries from the primary database, improving performance for the reporting application while minimizing application changes.

A standby replica is not a term used in RDS terminology. Multi-AZ configurations provide automatic failover but do not enhance performance for read queries. Thus, this option does not address the need for performance improvement due to increased queries.

While caching can improve performance, it typically requires more significant application changes to implement a caching layer. This solution is not as direct or effective for the immediate need to handle increased query loads as using a read replica would be.