Final answer:
To replicate user accounts quickly in Azure AD Connect, one should increase the synchronization frequency in the settings. This can make replication occur more frequently than the default 30-minute interval. Care should be taken to balance the increased load on the infrastructure with the need for rapid updates.
Step-by-step explanation:
To make Azure AD Connect replicate user accounts as quickly as possible, the correct answer is (a) Increase the synchronization frequency in Azure AD Connect settings. Azure AD Connect syncs data between your on-premises directory and Azure AD. By default, the synchronization happens every 30 minutes. This interval can be adjusted for more frequent synchronization depending on the requirements and capacity of your infrastructure. It is important to note, though, that very frequent synchronizations may increase the load on your server and network. Also, as per Microsoft's recommendations, you should not set the synchronization frequency to less than 30 minutes.