Azure SQL Database is the most appropriate offering for hosting a relational database that must support online transaction processing workloads for a multi tenant software as a service application.

Azure SQL Database is a fully managed relational database service that is designed for OLTP workloads and it provides built in high availability, automated backups, and strong transactional consistency. It offers scaling options that fit multi tenant SaaS, including elastic pools to share resources across many tenant databases and Hyperscale to support very large transactional databases.

Azure SQL Database also includes enterprise features such as row level security, dynamic data masking, and advanced threat protection, and it integrates with SQL Server tooling which is often required for complex transactional SaaS applications.

Azure Synapse Analytics is focused on large scale analytics and data warehousing rather than low latency transactional processing, so it is not the right choice for OLTP driven multi tenant SaaS workloads.

Azure Cosmos DB is a globally distributed NoSQL service optimized for massive scale and multi model document or key value workloads, and it does not provide the same relational schema, SQL Server compatibility, or OLTP semantics expected by many transactional SaaS applications.

Azure Database for MySQL is a managed relational option and it can run OLTP workloads, but the question context points to the Microsoft SQL Server compatible managed service with features like elastic pools and tight SQL tooling integration, which makes Azure SQL Database the preferred answer for typical multi tenant SaaS scenarios targeting the SQL Server ecosystem.

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