Which solution should provide the HIGHEST level of reliability?

Explanations:

While using Amazon RDS MySQL Multi-AZ enhances reliability for the database, storing sessions in Amazon Neptune (a graph database service) is not appropriate for session management, which can lead to increased complexity and potential reliability issues. An Application Load Balancer with an Auto Scaling group does enhance application reliability but the session storage choice detracts from overall solution effectiveness.

Migrating to Amazon Aurora MySQL provides high availability and durability with automated backups and replication across multiple Availability Zones (AZs). Deploying the application in an Auto Scaling group ensures scalability and high availability, while storing sessions in an Amazon ElastiCache for Redis replication group enhances performance and reliability for session management. This combination maximizes overall reliability.

Migrating to Amazon DocumentDB does not align with the requirement for a MySQL-compatible database. Additionally, using a Network Load Balancer instead of an Application Load Balancer is less suited for web applications as it does not provide advanced routing capabilities. Storing sessions in Amazon Kinesis Data Firehose is inappropriate as it is designed for data streaming rather than session management, leading to potential reliability concerns.

While an Amazon RDS MariaDB Multi-AZ DB instance provides high availability for the database, it is less robust than Aurora MySQL in terms of reliability features. Using ElastiCache for Memcached is a valid choice for session storage, but it lacks some advanced features and durability compared to Redis. Additionally, the solution does not enhance scalability as effectively as the one using Aurora, making it a less reliable overall choice.