You need to implement an Azure platform as a service (PaaS) service (DP-900)

Correct answer: A. Azure SQL Database.
Azure SQL Database is a platform as a service (PaaS) offering that provides a fully managed relational database service. It is designed to handle the management and maintenance of the database infrastructure, including tasks such as patching, backups, and high availability.

Azure SQL Database supports built-in autoscaling, which allows the service to automatically scale up or down based on the workload and resource requirements. This ensures that the database can handle fluctuations in demand without the need for manual intervention.

The other options are not the most suitable for this scenario:
B. SQL Server on Azure Virtual Machines: This option involves running SQL Server on a virtual machine, which requires more manual management and configuration compared to a fully managed PaaS service like Azure SQL Database.
C. Azure SQL Managed Instance: This is a PaaS offering, but it is designed for migrating on-premises SQL Server instances to the cloud. It may have additional features and capabilities that are not necessarily required for this specific use case.
D. Azure SQL Edge: Azure SQL Edge is a lightweight, containerized version of SQL Server designed for edge computing scenarios. It is not the most appropriate choice for hosting a relational database in a PaaS environment.

Therefore, Azure SQL Database is the best choice for implementing a PaaS service that hosts a relational database with built-in autoscaling capabilities.