Which database implementations will meet these requirements?

Explanations:

Amazon Redshift is a data warehouse solution, not a relational database for transactional workloads, and is not designed for high availability in the context of a mission-critical web application.

Amazon DynamoDB is a NoSQL database, which is not suitable for relational workloads requiring SQL or transaction management for mission-critical web applications.

Amazon RDS for MySQL can provide high availability with Multi-AZ, but it is not as fault-tolerant or scalable as Amazon Aurora.

MySQL-compatible Amazon Aurora Multi-AZ is a highly available and fault-tolerant database that automatically replicates data across multiple AZs, making it ideal for mission-critical web applications.

Amazon RDS for SQL Server Standard Edition Multi-AZ provides high availability and failover capabilities with automatic backups and replication to multiple availability zones.